Do not feed them bread or other significant carbs. That'll cut most the weight on its own. As for the term? Whenever they can fit through a simple thin gap in the fence, they can leave. Note the fence is also electrified, so you really can't force through and remain conscious unless you can actually pass without grazing it. By the time they have shed enough weight to fit and trained agility or pain tolerance enough to get out they'll have the discipline to stay thin.