to sub only
I only sub at one school and I am the unofficial "main" sub for that school. I have worked hard to get most of my jobs by request. I do work 3-5 days per week. At $64 (or is it $68) a day, I will only earn about $5,000 per school year.
I would not recommend substituting if you are depending on the income. My experience has been that besides not working during the summer, there are slow times at the very beginning and end of the school year, before and after holidays and school testing dates. Plus, you have to consider the number of days you will only work 1/2 days. As a requested sub, I will take most 1/2 days for teachers that normally give me most of their work. Then of course, a teacher who normally requests you may need you at a time when you are already committed to another teaching job. This happens a lot for in-service days as the teachers often are going to the same meeting.
On the other hand, if you are not depending on your sub pay, and you enjoy subbing, it can be a nice income. If you think about it, you can have a flexible schedule, pick and choose the classes you will take and so on.