Regardless of what the pitchers do or don't do during practice, they are at all the practices the whole time. The time commitment is pretty much the same. I know for weight lifting, on my son's team, pitchers go Monday, Wednesday and Friday mornings before school. Position players go Monday, Tuesday, Thursday and Friday. So, pitchers go one less day. Other than that, everyone is at every practice.
What do they do in practice? They throw every day. They run, sprints or medium distance. They shag fly balls during BP. I've even seen my son play 1st base for the infielders during BP. They'll do drills from the mound. Covering 1st base, fielding bunts, covering the plate on passed balls. All the things pitchers have worked on since they started pitching. They do whatever they can to stay busy. And did I mention, they throw? Once most of the general preparation is done early in the fall in terms of getting in shape, my son's team plays a lot of inter squad games. Their practices are generally around 3 hours a day. They'll start with getting loosened up. Then do the same things they'd do during pre-game. Take ground balls, take fly balls, take BP. Then they split into 2 teams and play a game. They'll do this probably 3 times a week.
In the spring, they are generally playing or practicing 6 or 7 days per week. Last season my son did not have a day off from baseball for the 1st 6 weeks of the season. It is a tough road and if you don't love what you are doing, you will quickly decide you don't want to do it anymore...