It depends on what and where you volunteer for. Some places are more relaxed than others about obligations.
I was a special constable for many years. The hours I worked and the type of work I did "flexed" enormously according to what else was going on in my life. That was fine with them at the time. We were given a list of duties up to two months ahead and we just chose which ones we wanted to do. We were also free to just turn up randomly at the station, alone or in pairs, and we'd present ourselves to the station sergeant who'd allocate us a beat to work alone or in our pair, or allocate us to go out on patrol with a regular. All absolutely fine. Some years, I did hundreds of hours of duty, others I did the bare minimum, which was "officially" ten hours per month, but in reality could be a lot less and some months I didn't go near the station - but made up for it later. The only condition was that we had to turn up for any duty we'd put ourselves down for - "no shows" were reprimanded, although in reality, it was just a telling off as there was nothing they could to punish you, short of taking away your warrant card and uniform.
All that changed when we got a new supervisor (regular training sergeant was the usual "supervisor" given responsibility for the specials). He was a right dick! Clearly wanting to make his presence felt. Started allocating duties, with no consideration to peoples' circumstances, which is crazy when most people had jobs, families and other commitments. He also went crazy imposing the 10 hour per month rule. Needless to say, there were many resignations, including myself!
I now volunteer in our voluntary-led village library (in fact I was the founder of the campaign to reopen it after it was closed). I do the occasional stint on the counter, but it's mostly back office admin/management that I do now. We have a good group of volunteers, who basically arrange the rota between themselves to ensure "someone" is there to open up, lock up, and man the counter. There's a blank rota pinned to the wall where people put down their choices as to which shifts to attend. Of course, sometimes shifts are blank and then myself or one of the other "unofficial committee" members has to step in to cover. But we never put pressure on anyone and we find that some volunteers just "drift away" and don't appear for several weeks or months, but when we notice that, one of us will do an informal "welfare check" just to check they're OK (Most are elderly), and we put no pressure on them to come back, we just say they'll be welcome back if and when they feel ready (whether physically or mentally or whatever).
I think people need to find what voluntary work is best for them. Some will have to be more formal due to the nature of the work, i.e. where H&S or training needs are essential, or where the service is relied on. Some volunteers prefer the more rigid approach more akin to a proper job, others want an informal option, more akin to a social life rather than work, where not turning up doesn't really affect anyone else.