I've been working on an indie game solo for the past four years. I've had quiet periods and I've had productive periods. But I have worked on it nearly every single day for those four years, rarely for less than an hour.

One method I did have a lot of success with was a time spent spreadsheet I created. For any given date I'd enter individual times I started and stopped working on it - so it might be 10am until 11.30am, 2pm until 5pm, and so on.

For each day I'd sum the hours spent, and then I had a target hours to hit each week - 20 at first, then 30, and eventually 40. This is while also working a day job, otherwise I'd probably have tried increasing it by 1.5-2x.

A key rule I followed was this: once I hit enough hours for the day to be on track for the weekly goal, I could then do whatever I felt like with my remaining time. Ironically I'd often keep working on it as the momentum was already going, but just as often I'd go relax.

Pretty good system all in all.