How long can I leave water out before I need to change it?

Once a day at minimum, and sooner if you notice dust, fur or food debris floating in it, which happens faster than most owners expect in a bowl near a litter box or feeding station. Water does not spoil the way food does, but it picks up bacteria from a cat’s mouth and from the air, and a film can form on the surface within a day even if the bowl looks clean. A cat who sniffs and walks away from a bowl that has sat since morning is often reacting to exactly this, not being fussy for no reason.