I would have to dig into my old bookmarks to be sure of the details of his old schedule, but if memory serves he would work an average of two or three days a week when he was doing the two part-time stations. His full-time schedule is 24 hrs on, 48 hrs off, and Thursdays always off even if by according to that rotation he should have been working. So by my figuring he's working roughly the same amount of days.
The shifts are longer, yes; part-time shifts are 12 hrs and full-time are 24. But a firefighter isn't "working" every. single. moment. of a shift. There are things needing doing at the station, but after 3:30 pm the firefighters are simply on call. They're otherwise free to do whatever they want. They have a gym room on site, Stipe can do some of his own strength & conditioning work, hell a coach can go there and hit pads with him etc. They just have to drop stuff if/when a call comes in. And sleeping is part of that 24 hrs, too...again, if not interrupted by a call.
Is it easy to juggle it all? No. But it's doable, and Stipe has had a lot of practice doing it. The only stretch there would be a real issue was during his first year of the full-time job when he was a "probie" (on probation), without flexibility of schedule, and mandatorily working extra shifts to fill in for firefighters who were sick. It was during that he turned down a fight (we know that because he's mentioned it in a couple of interviews). I take that as evidence that if he didn't think he had the time to do justice to a fight camp, he wouldn't take the fight.