There's a great progesterone cream thread in the alternative remedies section, I'll get that link for you in a minute. I would guess that there are times when you *could* use progesterone cream all through your cycle, such as if you are doing a "shutdown" to try to reduce cysts on your ovaries. I've never done that though and don't know a whole lot about it, so don't quote me on that. Since you are TTC however, it is not recommended to use it during the first part of your cycle as it can and does prevent ovulation. You would want to start using it AFTER ovulation is confirmed, either through OPK's or BBT charting. I usually start using it around 3dpo, and continue until my period comes. If you do get pregnant, you are not supposed to stop using the cream, as the drop in progesterone can trigger your body into thinking it's supposed to have a period, and you could likely miscarry.
Here's the thread:
New & Improved NPC (Natural Progesterone Cream) Info!
There's a lot of great info, and wonderful helpful women to help out if you have any other questions!