I was given a six month prescription for it and then when I had my review appointment and placed on the waiting list for injections, my FS told me to continue taking it as I at least have cycles on it. I was on it for 19 cycles in total. The reason they don't have you on it is it reduces the lining each cycle and most conceive in month 3 and usually never after month 6.
Many women who have had Ovarian Cancer have taken Clomid but the same could be said about other medications. The human body is a complex thing and each reaction is made up of a chain of other things. If a person who was taking Clomid develops Ovarian Cancer, chances are, they would have developed it if their body was 'normal' as all Clomid (and other drugs) do is mimic the things our bodies should be doing anyway.
If the benefits of taking Clomid didn't greatly outweigh the negatives, it wouldn't be available to be used.