I think this depends entirely on the nature of the startup.
If your startup requires consistently high turnover for new features, then it's going to occupy easily 40 hours per week on just that project.
However, if what you build doesn't need to be updated for months at a time (like some mobile apps or other software) you can probably get away with doing this. An entire company? I don't think so. A particular type of website, or app? Sure, if all you're doing consistently is maintenance.
If your startup requires consistently high turnover for new features, then it's going to occupy easily 40 hours per week on just that project.
However, if what you build doesn't need to be updated for months at a time (like some mobile apps or other software) you can probably get away with doing this. An entire company? I don't think so. A particular type of website, or app? Sure, if all you're doing consistently is maintenance.