I just have the one leo at the moment (plus a crestie that gets crickets occasionally as treats). The leo eats 3-4 large crickets every 3 days (her appetite went down bigtime when she stopped growing), so I end up getting 3 dozen crickets every 2-3 weeks, depending on how many of them die off on me.

As far as what you can expect, well, different animals have different appetites - some adults are like mine and don't eat a ton, others chow down whatever you throw at them and want to eat daily. Juveniles are pretty much universally ravenous, however, so if you start with a baby, you will go through a lot of crickets at first. (5-6 appropriately sized crickets/day, maybe more, you do the math.)