Currently the mentor dashboard says that to keep up with demand, each mentor needs to handle on average 20 submissions per week. That would mean five minutes per exercise if you spend an hour, which I think should be plenty.
Some difficult exercises may require more time and a bit more back-and-forth, but then on the other hand there are some simple ones in the beginning that often just requires a "well done and good luck on the next one".
Anyway, the system doesn't require you pledge any particular amount of time. As long as you can commit to finish coaching someone through an exercise once you picked it up, it should not be too overwhelming.
I copied your post to internals forum, hope you don't mind, just figured that posting this only in users forum is going to drive more traffic to learners taking the Rust track rather than mentors for the track.
Looking through my cpp exercises, I can see that I did 25 exercises on 4 days. Are the exercises grouped such that you can do a set until you need a response from the mentor? If I were constantly blocked on the asynchronous response from a mentor, I would personally be quite discouraged from continuing.
Maybe I'm just not the target group for mentors.
Edit: I see now that many of those exercises are part of the off-track set.
Someone doing that many exercises per day is probably fine for doing the track independently rather than mentored. You can always select that option. There are also non core exercises that open up after each core exercise is complete.
Is there any way to tell what the expected time to wait is before a submission gets looked at by a mentor? My first submission on the Rust track got looked at in the same day (which was great), but my solution to the next assignment has been waiting for almost three days now. In the mean time I've also done all the unlocked optional exercises. I understand that the mentors are volunteers, but I imagine you have access to sufficient data to make a reasonable guess about how long one would have to wait for a review, on average (even if the level of detail is just "hours/days/weeks" or "mentors for this track are active primarily during the weekend"). This would help set reasonable expectations. I for one don't mind waiting a week for a review as long as I know in advance.
There is an option to do independent mode, which opens all exercises to you. The downside of this is that your exercises will then not be mentored or commented upon at all. AFAIK there is also no way to reverse that decision and go back to mentored mode.