That all depends...are your cycles pretty normal? Do you usually O around the same time???
I started using my OPK's around CD9 this cycle, and here it is CD18, just finally got my first positive yesterday, and I was using 2-3 a day thinking I might miss my surger...so do the math...let's see...I used about 30 or so...but if you pretty much know a narrow range of when you should O, that should be plenty.
Myself, I did not have good luck with the internet cheapies. I started a thread over in the TTC board, comparing 4 different OPK...well heck, I'll post the pic for you here too. I get best results with the Answer ones you can get at Wal-Mart, it was like $11 for 7 or them, so they cost more....but if I were just using the others, I might have missed it..Here's a pic:
from top to bottom: Answer, Clear Blue, $Tree, and internet.
The answer is a clear positive, CB was iffy, $Tree not positive yet, and internet wasn't quite positive yet..even the control line was not really dark....but some people have great luck with them, so I think it just depends on the person..I think I just need more sensitive tests...