Quick question regarding watchers and services Options

[MazeFan12]

This is just a quick question regarding watchers spirits that came up this weekend and is kinda bugging me. Do watcher spirits have a set number of services determined by your successes, or do they merely do whatever jobs you can give them until their time runs out. I ask because I never popped into my mind that they would have a number of services. I can't really fathom why I didn't think of it, I guess it was because I had just read about ally spirits and I assumed if watcher spirits had a number of services it would have said in MitS. My GM disagreed with me, but did say he didn't know the rules that well and that was how his group had always done it. I figure he is probably right, but I needed to ask because it would have kept bugging me. In retrospect I wish I hadn't paid enough in ritual components to make the watcher last for a few months. Anyways, thanks for any feedback.