I don't know if I can even call it a side-project any more. I stopped working professionally 3 years ago.
Bought an e-commerce business that was making ~$2k/mo profit and ran it on the side while I had my DevOps job.
That project has now turned into ~$30k/mo profit, and I've bought two more e-com businesses that average ~$10k/mo between the two - these are early and I expect them to scale up once I smooth out a few bumps.
I go through phases of a lot of work, and next to no work. Depends if I'm busy on growing the businesses or just want to enjoy the income. I'm out of 95% of the day-to-day operations and I have a team of 3 that handle most things.