From the way Bon looks, at least a few weeks. And since she continues to give him half (or more) of her rations, she's bound to loose weight at a faster pace, too. Even moresoe since that's exactly what fat is supposed to be - the first source of emergency energy for the body when the usual nutritional influx is severely reduced. Thus she'd have a much better chance at surviving a period of hunger than, say, Bon who is already looking heavily malnourished.