10 years is an impressive time! I actually finished a campaign of my own a few years ago. I was 17 months of bi-weekly play at 4 sessions. So, about 136 total hours. They got to level 13. I designed that campaign to kind of wrap up around 12th level. In the end, they kept basically going everywhere except where they needed to, because they were avoiding the final confrontation. I had to give them a nuclear option to finally convince them they could finish it. There was room to continue on, but I wasn't sure it would be quite as gratifying.
Still, it was the first campaign I ever ran to completion. Like I said, I specifically designed it to end at 12, because that seemed a lot more achievable. I've never had a campaign go to 20. I'd love o do that, though.
I'm actually going to run that campaign again with a new set of players, and I'll more strongly entertain the idea of extending it past 12 if they want to. This new one starts on Sunday, and it'll be bi-weekly with 10 hour sessions. Yes, I have no life, and playing D&D is how I wish to fill my days.