Progesterone suppositories during ovulation?

Question for those of you who have used progesterone suppositories after mc's. I just had my 2nd loss last month and my doctor wants to try progesterone during this next ovulation. She told me to take them 48hrs after my positive ovulation surge. Well I got my positive Friday morning so I started today. My question is when do you stop taking them if you're not getting a bfp because I know progesterone can delay your period. Curious to know how many days past ovulation I should stop taking them if I don't get a positive pregnancy test. I want my period to still be able come on time if I'm not pregnant this cycle. 

Any advice or insight please!