[Gawku]

It (possibly) balances hormones when used daily, but isn't made to be used for more than 3 weeks at a time?

In the menstrual cycle, progesterone is only released significantly in the luteal phase, between your ovulation day and your next period. That's the only time you need it, otherwise you run the risk of creating another hormonal imbalance. It's no more than 3 weeks out of a month because your follicular phase doesn't produce much progesterone, if at all.
This chart shows you hormonal and menstrual timing every cycle:

http://en.wikipedia.org/wiki/File:MenstrualCycle2_en.svg

Luteal phase, and even a week beforehand if you choose, is the best time to use PC.

The safest way to go about using PC is to determine when your period occurs, if it's irregular it's obviously more difficult. If you google an ovulation chart website and put in the date of the start of your period, it'll let you know when your ovulation is (usually 2 weeks from the first day of your period). It's really complicated stuff, but balancing your hormones requires a lot of research and overall knowledge. When you figure your body out, you're more likely to get the results you want.