Short answer: You don't know. There is no shortcut. But if you want to give it a shot, you have to go all the way:
1. Build it for yourself.
2. Start using it for yourself.
3. After a fairly stable release for yourself, start putting it out there for others to try.
4. Work on marketing it (Hardest step. Most people give up at this step). For marketing, learn how to do "inbound" marketing vs "outbound" marketing.
5. Get a professional looking website, let clients signup for a trial. Talk to them on the phone. From this step on, it is hopefully no longer a pet project.
6. Provide support to customers and take their feedback for improvements.
7. Keep making changes to the app based on client feedback. Rinse repeat
There is no fixed timeline to try this out. Depends on your market and