The general notion of the ripple effect, of changes to a timeline taking time to propagate themselves through said timeline, makes sense in a certain way. (At least it's able to followed easily by the human mind, which is important for fiction.)
The problem is that it isn't used consistently in Back to the Future. Note how, for example, Marty has a week before he erases himself from existence, whilst in BTTF II, the newspaper and Biff's matchbook change instantly before their eyes.
Either it's instant or it's not. Inconsistencies like these annoy me.