wow - they go alot don't they ? I'm moving from a 3 bdrm house to an apartment and my girls are also used to being let out whenever they need to be - so I'll be watching this - (mine don't go that often if this is any help - They pretty much pee whenever - but the other is usually once a day)
When I'm at work they've been pretty good about either using a pad or waiting till I get home - BUT - they don't eat or drink hardly anything when I'm gone - they're too busy with their little noses at the door listening for my car
it COULD be the food you have down for yours - maybe try another brand ?