The longest a legacy character took over for an original was Wally West taking over Flash from Barry Allen. That lasted more than twenty years (and never should’ve been reversed imo). Others range from a few months (Ben Reilly, Azrael, Jim Rhodes, Johnny Walker, etc) to a few years (Kyle Rayner, Connor Hawke, Jaime Reyes, etc). Maybe they would’ve kept Ben until the Spider-Man movie until 2002, but Peter Parker is so synonymous with the Spider-Man identity, I really don’t think it would’ve been sustainable in the long term. And there were not willing to commit to Ben taking the Peter ID as it made no sense in-story for Ben to do that while OG Peter was still alive.