The opks detect your surge, then you are supposed to ovulate within the next 12-36 hours. I have used them opks, along with charting my bbt for the past two months, ttc for five months total. I know that I am ovulating by my bbt, but no luck conceiving. :(
to answer your questions, your chances of conceiving are 25% each month. I would recommend charting your temps to make sure that you are indeed ovulating.
Good luck!