I have a kid, a life, and I running my start-up and still there is enough time for pet projects. I do pet projects mainly on weekends, not the entire day, but a few hours of it. I also do a few hours at night, but that is rare.

You don't need many hours, you need to make a good use of the hours you get.

For example, I know that I will have only 3-4 hours on Saturday and Sunday. If I don't do what I need to do, I will have to wait another week. As a result, I am really focused on the core of my pet project.

This works for me. I know people that prefer doing it a few hours a night, but I rarely can. At end it is just a question of focus and organization. If you do those two things well I am sure you will find the right amount of time for your projects.