I water roughly 2.5 litres every 5 days now and wh...

Hi @Stinkygorilla! Seems fine, I usually water with ~20% of the pot/container size. So in your situation, considering you're using 40L containers, I would use 8L. This should flow to the bottom of the pot, and give you a little drain (which can be useful if you want to check ph/ec). However this basic rule does not always apply, sometimes they drink more, sometimes they drink less. Keeping a close eye onto your plant's health will help in finding the perfect amount of water depending on their current stage/health/temp..Hope this helps, keep us up-to-date and happy growing!