I've been writing a book last month, dedicating only one hour a day on it. That is a small amount of time and you can allocate it easily. Although I missed days due to different reasons, It's hard to find an excuse for that. The beauty of this approach is that I've got a lot of work done without big efforts.
One hour seems insignificant, but it isn't true. It total, I worked for about 30-40 hours, which is one week of working full-time. But I'm sure, If I spent the same amount of time in one week, I wouldn't have the same progress. It would be much less.
The trick is small steps and micro deadlines. When you sit down for only an hour, focus comes easier. Because the time you have is limited, it works like a deadline - you must do as much work as you can within a short period of time. At the same time, I haven't been trying to turn it into a habit. I missed some days and didn't want it to become routine. Creativity is often impulsive and it's important not to burn out. When I didn't want to write a book, I switched to another project.
This method is flexible and efficient. I call it "creative sessions". Each creative session is a small step towards a big project, the project that I hope will change me in some way.