And it also depends on what conditions the plant is in.
-How big are your pots? If they're small, they dry up much quicker (i've had to water plants every 4-5 hours).
-What kind of lights are you using and what does your grow space look like? If the grow space is too small and gets too hot, the water will evaporate faster from the soil.
-Are you using a fan? A fan will dry out close pots that it's blowing directly on much faster than normal.
These are just a few things to consider when evaluating your grow. You can also go to Wal-Mart and buy a $10 moisture gage that you stick in the soil. That will be a good indicator whether you should water or not. Much more convenient and accurate, and less damaging to plants than using your finger.
Hope this helps!