How long does a distribution upgrade normally take?

I recently hopped a server from 9.10 to 11.10 (4 separate upgrades) in a couple of hours. There was significantly less to do on each upgrade and it was downloading at a stupidly fast speed but that should give you a lower bound.

Upgrading does take longer than reinstalling. Not only do you have to download the packages but they have to extract and configure themselves, running all sorts of nonsense to ensure stability. Most installed systems tend to have more packages than a fresh install too.

But what you're going through does not sound normal. I'd watch the terminal view for a little while to make sure it's not doing the same thing over and over and over again, if it is, you might need to put together a Plan B.

If it looks like it's working properly - just leave it as long as you can bear and see what happens.

I just upgraded from Ubuntu server 19.10 to 20.04 running on DigitalOcean with minimal resources and it took about 30 minutes.

I would just not recommend running the do-release-upgrade command via SSH.