If you have not checked the exam timetable for next year, I would strongly advise you to take that into account as well. I did not do so, and ended up with 6 hours of exams on one day, which really is a waste of effort, since I could feel my grades drop in that last hour of exams. Some people had more than 6 hours, so would have had to stay over at a teacher's house, but they decided to drop a subject in May to avoid the stress of all that. Maybe finding a combination of AS subjects that gives you 6 hours in one day may swing your decision if you are on the margin.