The 4/4/4 Rule
The first thing to note is that I have two "jobs." My main source of income, first and foremost, is making music. Make no mistake; writing songs is not a hobby for me. My ASCAP checks are how I pay my rent and bills. My wife runs our record label, so I don't have to deal with that, for the most part, but a constant steady output of music is required to make a living when you're not on a major.
Audio Damage, which is my other "job," is actually starting to eclipse music making, in both time and income. However, it's only really time consuming for me when we're designing a new plug-in. Adam does the lion's share of the coding; I do the user interface and the editor, then write the presets, while he does everything in between. I also do all the support and the business side of things. So it evens out in time spent, but his comes all in one chunk, while mine is a more consistent daily regimen. 90% of the support involves answering the same questions over and over again, so that's not too taxing, time-wise. Tedious more than anything else.
So, keep in mind that when you work a normal 8-hour-a-day job, you also spend an hour getting ready and going to work, and an hour coming home and reorienting yourself to not being at work. So an 8-hour-a-day job actually takes at least 10 hours out of your day. If you sleep 8 hours, that only leaves 6 for everything else, and that's not enough. You have to spend those 6 hours coming up with reasons to go back to work the next day, in my limited experience on the subject.
When you're self-employed, there's a real danger of spending 20-hour days on your pet project, to the detriment of everything else. This is not only a recipe for burning out, but it actually hinders the process. So I practice what I call the 4/4/4 rule. I NEVER allow myself to work more than four consecutive hours on a single project. So my day will be split up in to four hours of working on Audio Damage graphics or whatever, four hours of writing music, four hours of working on some DIY thing, and then the rest of the day, I'll read or watch TV with my wife or something. As long as I do that, I'm able to keep things in their compartments, and no one thing takes over my day and becomes too much like real work.
The final key is that I never spend more than 12 hours a day working, period. Even if I disregard the other rule, and don't limit myself to 4 hours on a single project, I still never work more than 12 hours in a day, even if I have to force myself to stop.
I don't know if this is illuminating or helpful in any way, but if you're curious as to why it seems like I'm a monkey on meth, that's how I pull it off.