It should be changed once a day at a minimum, because bunnies get bored with the "same old stuff" sitting around, and get excited at new handfuls of hay. It's best to give smaller portions of hay more often a day, instead of dumping a single heap to last them the whole day. Personally I like to change my bunnies' hay 3 times a day - in the morning, after I come home from work, and right before I go to bed.
Yes, once a day is fine, just don't leave damp hay over 24 hours. Discard the remainder and use it for compost, it will make good plant fertilizer or potting soil.