There is no "takes hold" - the timeline is changed the moment he returns to 1955 and hands over the almanac. From that point on, as explained by Doc, the timeline can only head to the alternative, dystopian version and if you travel forwards in the DeLorean you can only go that way. It's the reason given that they have to go back to before this happens to keep the original timeline intact.
My point was that due to the change in 1955, when Biff returns to the future, it starts to change around him. This is in keeping with the first movie, when Marty gets hit by the car (his dad is a 'peeping Tom') and accidentally changes the timeline, the changes
aren't instantaneous. He has time to fix things before he vanishes at the concert- the process begins with his family photo changing... 'poor photographic paper, your brothers head is missing' etc.
Thus when Biff hands the almanac to himself in 1955, he can return to that 2015 because the new timeline hasn't been established yet so there's a 'grace' period. That happens when 1955 Biff makes his first bet using the almanac. If what you say is true, then when Marty gets hit by the car, the change would have been immediate and he would have vanished, probably creating a paradox since its his time travelling that caused that situation in the first place.
So the entire first movie is a big 'grace period' in which Marty must readjust the timeline or he's gone....