In a walkover, there is no way to get 48 minutes for your five starters. If you have a different player at starter, backup, and reserve, the breakdown is something like:
starter: 36
backup: 10
reserve: 2
(I don't remember if those are the exact numbers, but it's something like that.)
If you put one player at all three spots, he will only get 46 minutes (again, not sure if that's the exact number, but it's close) - it'll be slightly less than a full 48 minutes.