The amount of time it takes to run an online community is misunderstood. It doesn't matter how many hours you work. Many of the best online communities are ran by people working just an hour or two a day.
What really matters with time is which hours you work and what you do in those hours.
1) Which hours should you work?
If you work from midnight to 8am, you’re probably not going to be an effective online community manager. All your members will be asleep. It’s better to work when your community is most active. If your community is only active for 4 hours a day, you probably wont need to work 13 hours a day.
2) What do you do with your time?
You can run a really successful community in just 1 hour a time if you know exactly what to do in that hour. It varies from community to community, but if you delegate the day to day stuff to volunteers (beginning discussions, replying to comments, recruiting volunteers, organizing events) and focus on activities which develop your community.
The time you spend working on a community isn't irrelevant, but it's not the decisive factor. Plan your time and activities very wisely. Focus on the tasks only you can do; the tasks that require special access to your organization or relationships with key members.
