I agree get the books if you haven't they are well worth the money http://www.amazon.com/Corn-Snakes-C...0706/ref=sr_1_1?ie=UTF8&qid=1325474275&sr=8-1
Honestly I give mine fresh water when I feed them, which is once a week. All mine are perfectly healthy and happy.
Once a week... but I check more often for dry bowls.
It is getting really dry here in the winter.
I dump them, rinse them with vinegar, wipe them out and fill them.
With 150 snakes, I don't think I could do it every couple of days!