What I learned from other people who have success in their side projects is to choose a workplace that allows you to do a sub 40 hour work week. Of course you would have to live a tad more simply than your standard upper-middle class software guy.
It turns out it’s hard to completely drop projects in the spring/summer and pick them up in the winter. I’d love to keep pecking at them over the photography season but I have a hell of a hard time getting anything done when I can only dedicate an hour or so an evening to it.
Actually, many software jobs are like that. All it requires is for you to not be ambitious to create. You can have very good salary in many companies while slacking a lot. Sometimes you might have to be in office for a lot of hours, meaning doing leisure, socialization and side project in the office.
The specifics will vary greatly depending on your employment agreements and legal jurisdiction, so make your own decisions regarding your own situation (doing it on company time might be the lesser risk compared to not doing it at all), but do be wary.
If you’re salaried, you’re always on company time, so whether the company owns it or not is up to the laws of the state and the agreements you signed when hired.