Everyone can find 15-20 hours per week. It's a matter of priorities and choices.
A 9-5 job is going to take up 50-60 hours per week, including prep time and travel (possibly less or more in some scenarios). 50 hours for sleep. That leaves a very solid 50-60 hours per week, that is left to allocation by priority (some will go to food, life maintenance, etc. - that all has a blend of choice and priority to it).
How can you not shave off 15-20 hours per week out of those 50-60?
Pursuing something like this should be assumed to be difficult and brutal. The world will fight you (the world, made up of people, does not like change), people close to you will reject your efforts (you'll be lucky if 5% of the people close to you are openly supportive) and you will suffer. A world filled with a crab mentality will try to kill what you're doing. Conform, give in, give up, quit, stay in your miserable 9 to 5 that you don't like, be one of us. It's part of the deal: you will have to suffer immensely for what you want, in most cases. If you want it, you have to absorb that.
So how do you handle it? You must want the thing you're pursuing more than the pain it's inevitably going to entail.
Steve Jobs has one of the great quotes on this matter, from the D5 conference in 2007 (say what you will about him, he plainly understood what he's talking about on this subject):
"People say you have to have a lot of passion for what you’re doing and it’s totally true. And the reason is because it’s so hard that if you don’t, any rational person would give up. It’s really hard. And you have to do it over a sustained period of time. So if you don’t love it, if you’re not having fun doing it, you don’t really love it, you’re going to give up. And that’s what happens to most people, actually. If you really look at the ones that ended up being “successful” in the eyes of the society and the ones that didn’t, oftentimes it’s the ones [who] were successful loved what they did, so they could persevere when it got really tough. And the ones that didn’t love it quit because they’re sane, right? Who would want to put up with this stuff if you don’t love it? So it’s a lot of hard work and it’s a lot of worrying constantly and if you don’t love it, you’re going to fail."