If you do have periods, use it only in the second half of your period, the last 14 days. If you find yourself pregnant, I would continue using it for the first trimester. The sudden drop in progesterone when you stop using it could possibly cause a miscarriage, if you stop it when getting a pos pg test. Instead, phase it out slowly at the end of the first trimester, when the placenta starts taking over progesterone production.
If you do not have periods, or very very long in between, try using the cream for 2 weeks, stop for the next 2 weeks and repeat that a few times to see whether that gets your body back on track.
I would recommend you to get the book 'What your dr may not tell you about premenopause' by dr Lee. It has tons of info on progesterone cream and about the whole female hormone balance thing. I wish I had known about that book sooner.
I think that progesterone cream helped me to balance my hormones and got me pregnant with a keeper this time. Of course, I don't know what would have happened if I hadn't used the cream, but it is quite a coincidence that this happened a few months after starting the cream.