Once is plenty, and then only in an emergency. The more of the same genetics getting recycled, the more you're going to end up with some pretty strange calving conditions (if the calves live at all). Its always been my personal opinion to leave the linebreeding to the huge seedstock producers who have the time and the documentation on hand to do it with any hope of success (unless a person is in a really tight spot, but that's a personal decision that shouldn't be made lightly).
If you really like the bull's genetics, I'd cross his daughters with an outside bull that complements what you're looking for, and then cross back their heifer calves to the original bull. That would be all I'd do though.