I'm just getting started, but I currently have 3 side-startups (more like side projects) I'm working on. I found that I needed focused, full-time work in the initial stages. Once the website reaches MVP, and has a user base building up, I am able to improve it on the side.

Using the 80/20 rule, 20% of the time you spend building the project will achieve 80% of the use case. The rest of the 80% time is spent on closing the 20% use case gap.