Provera questions

I was just prescribed provera monthly to help with my irregular cycles. What I'm confused about is that inwas told to take it for 10 days starting on the 1st of every month. Does that sound right? Wouldnt that mean I'd have a 40 day cycle each month? Super confused. They wanted to put me on birth control, but I dont want that, so this was my option. They mentioned that it might help with fertility??? But how do I know when to take it?