Each subject gets 5 50-minute lessons, but if it's period 5 it includes registration so I think those lessons are 65 minutes long. Last 2 lessons on Wednesday are Enrichment, although I do my Enrichment Monday 6 and Wednesday 5. 1st lesson on Fridays are sort of citizenship lessons. As there are 6 periods per day, there are 30 per week. So 30-(5s+3) where s is the number of subjects gives the number of frees - in my case, 12 frees per week.