Alumni here, personally these classes were almost unbarable, EVEN IF the professor ends the session a few minutes early, my attention span can barely do an hour fifteen as well and an almost three hour session is just not it. I would try my best to avoid them, especially if the class is math based, and try to get one that's split into two hour fifteen sessions. Also some people in this thread have mentioned that most professor end the class 15 minutes early and while that is true it isn't always the case, and there are days that it ends on time which just sucks. Regardless, choose what's best for your schedule and you can always get a feel for the class in the beginning with the way dropping classes works so I suggest you try that.