How much time it takes up will likely depend in large part on what your actual duties in those two small classes are. If you're mostly grading papers/exams (and recording the aforementioned grades), then unless the instructor is having the students conduct multiple writing assignments, it shouldn't be too bad. If you're having to come up with your own lesson plans, that will take substantially more time. If you're teaching, but will be doing so from a pre-determined plan, then it will probably require a time investment somewhere between the above two scenarios.
If you're also having to hold more than a couple office hours per week, with two break-out sessions, my gut reaction would be that it might take a bit more than 20 hours/week if you're doing much more than grading and answering emails. Then again, checking with current students at your program is probably the best way to get an accurate estimate.