I don't know. We're supposed to get updates for a long time, so I suppose it's likelier I'll be done a long, long time before I'm finished.

I'm already 650 hours in, and that's with slowing down the last few weeks (nothing to do with coronavirus, I'm disabled and always play AC like this - in older games I just began TTing, and that was heavenly to me. Don't know if I'll ever have that experience with ACNH).
I enjoy making rooms in Harv's Island. If we get more furniture, I can see myself doing that and dressing up villagers a lot.
I also love hanging out with the villagers (it's one of my favourite parts of the game). If they add depth to those features, I might hang out with my villagers sometime. In previous games, once I was done, I was done, because of the fear of turning on my game and seeing somebody had left and/or moved in. That's not an issue any more, so I'll be able to come back.
There's always terraforming, can make a really ambitious island or something. It's unlikely I'll do that until I'm finished, if only because I'm attached to how my island currently is.
If they have minigames and things, I might play those with my partner. I might trade online, help newer players, just hang out on forums until we get another AC. By the time I'm finished, I imagine a lot of players will be, so maybe things like roleplaying, mafia, etc. will be big again, on forums and in game. Maybe go back to playing a different AC game. I'd like to say I'd get another copy of the game, but, well... My save file will always be precious, especially a finished one, and it's one per Switch, so I guess I won't play the game again. ¯\_(ツ)_/¯