My colds typically last 3 weeks with about 7-10 days of them being full-on. I also have an (obviously) terrible immune system and get quite a few per year.

Obviously it’s not doable to stay away from school that much, and I only did when I had a fever or was puking. That said, there were probably some years I at least got close to that 18 days. I would think the few years I went on a vacation I definitely went over it.