I've never kept anything wild. Normally I fed it until it was well enough to go on its own. I've kept several ducks and geese for several weeks just until I could drop them off at a nice pond with others of their kind.

I've known people who kept fawns and raised them until they were full grow deer. It always led to heartache though. Either the deer became jealous about whoever they viewed as their friend or it would get shot during hunting season.