I only have 10 units, and when they handed me my brand new timetable, I technically had 30 free periods a fortnight. That count includes the 5 "Period 0"s that start at 8 am (which I obviously don't get up for when I don't have them) and the 8 after-school peroids that no one ever sticks around for. There are some timetabled classes then, as well as some extention classes, but I only have 2 period 5 classes each week. There are also a four days I can go home at lunch.
If I take away the free periods that I don't spend at school, I have 13 frees. I then have to allocate 7 of those for my OHS French Continuers course, and 4 for my OHS French Extention Course. That leaves me with 2 actual frees a fortnight that I have to spend at school. I usually spend them properly, doing my french work. Mostly because I feel slack for having such a flexible timetable, but also because most of my friends spend their study periods properly.
This is only for the remainder of term 4, though. Next year I get a brand new timetable that I have to figure out all over again...