As long as possible. I have a couple of projects:
https://words-keeper.com - 10 years old. I even participated in the Google Cloud Platform Developer Challenge 2013, though I wasn't selected as a winner :( I was very upset because a lot of winning apps were much worse than mine. I suspect that 99% of those winners are closed now.
https://unilist.app - almost 5 years old. I developed it in April 2019 and have been using it almost every day. I started sharing it just a couple of months ago, perhaps it's too late, because there are tens or even hundreds of note taking apps are available now.
I don't have a lot of users at the moment, but I have a lot of ideas :) so I will keep improving both projects as long as possible.
I manage my projects, keep notes, tasks and different lists in my app https://unilist.app I’ve been using it almost daily since 2019 and started sharing a couple of months ago.