Budgeting time is just like budgeting money. You just have to make sure you're spending it well. It's best to be diversified, so spend some of that working time on something which can bring in an alternative revenue stream. It may not pay off right away, but it could pay off huge in the end.
I think it depends on the type of work. We're developing software, and this can be done practically anytime. Personally I go to office very late (to have some time with my kid in the morning), and often return early enough to also play in the evening. Then, after putting her into bed, I can work again until 02.00 AM. But it's also important to mention that your wife (or spouse) has to tolerate this, too! :)
And to side projects: I think they're even more important the more time you invest into a certain single project, to look beyond one's own nose. They keep you open-minded, and sometimes even integrate into your current main project.