I don't think there are ever rules in place for when a QB should be taken out.
I definitely do not see any logic to saying that no matter how a QB plays, you stay with him the entire season.
Losman was a victim of nothing other than his own poor play.
So I really don't hope anything of the sort regarding keeping a QB in.
I just hope that Edwards proves to be a better QB than Losman, because Losman is a bust.