Originally Posted by Jennifer
Marty didn't fade in a couple of hours in 1955, despite the fact that he eliminated the chances of his birth. It took a week for him to fade because he was still in the right time period to fix the discrepencies.
There's two ways to look at this:
1) Marty and the DeLorean were in 1986 when Emmett Brown (Citizen Brown) was still alive, so they were in the right time period to fix the discrepencies.
2) Marty put the spark in Citizen Brown's mind about the time machine, so time was still being written in regards to the DeLorean.
Once Citizen Brown fixed the time machine, the discrepancies in the timeline (that being that the DeLorean never could have existed since Emmett Brown never built it) were repaired.
What? You can't fix something that DOESN'T EXIST! No amount of repairing can stop something from fading out of existence. It's not possible.
The reason why it took a week for Marty was because the dance was the junction point. The junction point for this change is the science expo which was long ago. Plus there is no reason for time to catch up with Doc instantly but longer for Marty.