If you’re growing cannabis indoors, how often should you water it?

When to Water

You need to maintain proper water balance from the very beginning - before your seed has even germinated into a seedling. Since the seeds don't have any roots, they will die if they dry out, but they will also drown if there's too much water. When it comes to watering your marijuana plants, some factors will determine how much and how often you should be watering. These factors include the size of the plant, the size of the container your plant is growing in, the canopy, the root level, the temperature of the grow tent, the humidity level, and which stage of growth your plant is currently in. A good rule of thumb is to wait until the soil is drying (but not yet completely dry) and then water it enough to last it until the next scheduled watering.
For soil growers using standard 7-20l pots or containers, the best advice is to lift the containers. Dry plants will be relatively easy to pick up. Wet containers will be heavier. Assessing by weight is the easy way for the soil grower to figure out the wet-dry cycle.
As for hydroponic and coco growers utilising automatic watering systems connected to a reservoir, it is not so simple. Lifting up plants may not be practical or helpful. If you're running such a grow op, odds are you have some cultivation experience under your belt.
In this case, it's best to monitor plants behaviour post watering. Watch for warning signs and with a little trial and error, most growers get the wet-dry cycle down. The other interlinked element to consider is the air/water mix. Remember check pumps and feed lines regularly. The reservoir must contain oxygenated water and be free from pathogens.
Ganja farmers sowing cannabis seeds in the earth must assess the soil prior to planting. Knowledge of the pH value and how well the soil drains or doesn't is invaluable.
Amendments can be made to improve the soil if required. Rain is to be avoided, cannabis can foliar feed but the grower must control the schedule. Invest in a greenhouse or at least build some DIY plant cover.
Outdoors, the grower will need to scrutinise post watering behaviour of the crop. Likewise if growing in very large 30l+ containers, that can be heavy, the same advice applies.
Sometimes high temperatures can be unavoidable indoors and outdoors during the summer months. Excess heat will, of course, make cannabis plants thirsty and require extra watering. The grower must be flexible and read the signs cannabis plants express when they need water.
How much should you water cannabis plants?

The amount of water your plants need will depend on a number of factors, including:
Size
Outside temperature
Overall health
Stage of growth

You want to soak the pot and have run off through the drainage holes in the bottom of the pot. Water should pool up on the surface of the soil while you're watering, but it shouldn't sit on the surface after you move on to the next plant.

If a plant is very dry, water will run straight through the pot and quickly come out the drainage holes. If this happens, water the plant a little bit and come back to it after 10 minutes or so and water it again, and maybe even a third time. This will allow the soil to slowly absorb water incrementally, until all of the soil is thoroughly wet.

Take notes, make calculations, and get your plants on a watering schedule. Setting a cycle where the plant needs to be watered every two to three days is ideal.

As the plants grow, so will their need for water. You might need to top-off individual plants in between their full waterings as they grow and you figure out a consistent watering schedule.

The sooner you find the sweet spot between too wet and too dry, the sooner you'll see your garden flourish.
How to Collect Runoff Water

It's important to keep plants on saucers or trays so you can remove the runoff water. You can collect the plastic saucers one by one and dump them out, but that also gets inconvenient with many plants.

It's inconvenient to empty saucers one by one if you have a bunch of plants, but you don't want to leave plants sitting in runoff waterYou don't want to have to empty your saucers one at a time when you water your plants

Plants saucers have to be emptied one by one

If you put your plant on plastic trays, and then put the trays on a slight incline by putting something small underneath in the back, it will catch all the runoff water and cause it to drain to the front. The item in the back only needs to be about half an inch thick, for example a piece of plywood. However, if you can find something more water-resistant, like plastic, that's even better.

These 1'x2′ plastic plant trays work well if they fit your space. You can fit four of them in a 2'x4′ grow tent (this is the grow tent I use) with up to two plants each as long as your plant containers are 11″ wide or smaller at the base.

Put trays on a slight incline by placing something underneath the tray in the back. This causes all the water to come to the front for easier collection. Each of these trays has a small plastic board (which we found around the house from something else) under the back. Anything that's about half an inch high will do the trick. These particular trays accommodate plant containers up to 11″ wide at the base.
How Frequently to Water Cannabis

Seedlings dry out quickly and therefore need a lot of attention. Depending on the environment, seedlings may need to be watered twice daily but only with a small amount of water each time. Larger plants, on the other hand, should be allowed to dry out slightly - but not too much! This gives the roots a chance to breath and expand as they look for a water source.

It's important not to let your plants go too long without water. Though some may be able to recover from drought, excessive dry periods could cause flowering plants to turn hermaphroditic.

Generally, larger plants should be watered every 2-3 days in the early morning (or as soon as the lights turn on for indoor grows) to improve nutrient absorption and reduce the chance of mold. Remember that the frequency of a watering schedule will vary by grow medium and environmental conditions. A good rule of thumb to determine if your plant is ready to be watered is to your finger in the soil up to your first knuckle (around 5 cm). If the soil is dry, it's time to water. If the soil is still wet, hold off for another day.
How often do you give your cannabis water?

Well, you will want to water your marijuana whenever the top of the soil or growing medium starts to feel dry. I like to water when the medium feel dry up to my first knuckle, or about an inch.
Soil - Water plants when the soil feels dry up to your first knuckle (or if the pot feels light).
Coco Coir - Aim to water plants every 1-2 days. If coco is staying wet for 3+ days, try giving less water at a time until plants get bigger and start drinking more. Don't wait for your coco coir to dry out, but don't water if the top inch feels "wet". If the container feels light, it's definitely time to water!

How to water cannabis properly (when using a well-draining potting mixture with liquid nutrients)...
In soil, wait until the topsoil feels dry about an inch deep (up to your first knuckle - just use your finger to poke a hole in the soil and see if it feels dry).
In coco coir, you want to water every 1-2 days if possible and adjust the amount of water you give accordingly. The top inch doesn't need to completely dry out between waterings.
If you're regularly adding nutrients in the water, give enough water each time that you get 10-20% extra runoff water drain out the bottom of your pot. This prevents a buildup in the potting mixture because otherwise, you are continuously adding more nutrients to the system.
Go back to step 1. Note: If water takes a long time to come out the bottom, or if pots take longer than 5 days to dry out before the next watering, you may actually have a problem with drainage (more info below) or need to give less water at a time. If your plants are very small compared to the container they're in, give water more sparingly until plants get bigger.