There are three points at which you can quit building a community.
The first is when you fail. There’s no shame in failing, you tried an approach that didn’t work. You have more knowledge than your competition. You’re one step closer to succeeding.
The second is when the community is self-sustaining. You realise that you don’t need to stimulate the activity anymore. You can begin winding down your work, recruiting volunteers, delegating responsibility until you can move on to your next project. Don’t be tempted to make a big ego-pleasing announcement, just slip out the back door. Job done.
The final option is to do the opposite. Rather than doing less and less until you’re irrelevant, you can decide to do more and more until you’re invaluable. You can reach out to more groups. You can try to secure more press coverage. You can bargain for more from your client (yes, it usually is a bargain). You can build new relationships. Arrange for bigger meet-ups and events.