Twice a day (and they watch the clock, little pigs) with some treats for when they go outside and poop/pee. Though they try to convince me that if they just WALK outside, I owe them something.
If I left food out, they would eat until they threw up and then would eat again. They got into their food once and ate about 2 pounds (they're only 20 pound dogs). Their stomachs were distended and we were ALL miserable until their systems were cleared out.
I feed them the NutriMax overweight stuff and then some kind of absurdly expensive treats.