I think it really depends on the type of side project.
Trying to start a small business in your free time? Probably a lot of hours. I wouldn't be surprised to see people spending 20+ on the weekend for this.
Trying to beef up your resume for an imminent job search? Less than the start your own business guys, but a good amount. Maybe 10 hours on the weekend.
Do you just really, really like programming? No limit here then. As much time as you can while still eating, sleeping, and keeping your spouse happy.
Is it just like any other hobby to you? Probably <10 hours in most cases. It's not like you have any reason to finish in a timely manner and you probably have other hobbies to get to.
Have a 9-5 tech job that you plan on staying in for a while? Most people in this situation are probably at 0 hours. For most people it's just a job. No need to bring it home. Also, once you have kids and other responsibilities side projects will be one of the first things to go.
Anyways, my point here is that unless you're trying to monetize it somehow, either by starting a business or getting a future job, just do it however much you feel like. 0 hours, 10 hours, 20 hours, it doesn't really matter. It's supposed to be fun. There's nothing wrong with sitting in front of the TV all weekend, or camping all weekend without any technology, or doing anything else you want on the weekend.
A lot of people on this website have drunk the productivity kool-aid, some for good reasons, but it tends to be a little extreme in my opinion. There are better things in life to be than productive.