the best truth teller is ur plants self....if u c they getting yelllow u know ur watering too much..if u see they greenish and pointing upwards know that they r doing just fine..and if they green and pointing downwards know its time to water them..but most important for me is plants need to drink evry day or 2 days..but twice a weeks will put me on a risky side. but hagis when u talking seedlings we hv the same pots and i always water evreryday..i use 1,5 l water fro 3 18l pots since seedlings direct germinated in the pots. check out my seedlings..they r on a 11 days veg today and watering everyday