If they are still gaining weightsteadily, I would wait till thre weeks. If their weight gain is slowing down, or evening out, I would start at 2 1/2 weeks. Start with one meal a day at first for a couple of days, then add another meal everyday or other day until you're up to 4 meals a day.

They don't eat much to start with anyay, so that way, by three weeks, when they are really starting to need it, they'll be getting that little bit more.