Completely agree on not setting a deadline, it will likely lead to disappointment as we tend to set unrealistic deadlines anyway, even more so for personal projects which rely on having time off of work and other responsibilities. I would even skip the "define a goal and a scope" part. Just go for it and see where it will take you, it might go into a totally different direction, which is fine.

I started recording songs for a Christmas album in 2007 and just published them this month. I thought I would never release them. And every time Christmas was over, I didn't feel like working on them anymore. So I only had about a month per year to occasionally work on them. There were years when I didn't touch them.

So after 16 years, I could see the home stretch and I decided to make it to the finish line. (I recorded the final music video today, actually.) If you had pushed me to name a deadline back in 2007, I would've said 2008.

I have a whole bunch of "projects" lying around, some of them are just ideas at this point. They will get finished eventually. Or not. No pressure.