I usually have 1"-1.5". This is for several reasons.
1. The beneficial bacteria that will inhabit the gravel bed only goes down to about an inch or so....and
2. Any more than this and all your really doing is giving room for all the gunk (detritus, uneaten food and so on) a place to hide.
If you are going to have live plants I believe an appropriate amount of gravel would be aroun 2-3" so the roots have enough base to take hold to.