From what I understand it depends on why you are using it. Many people use AC to help regulate their cycles and make them less painful so hormonal rather than fertility reasons - in which case I think you are supposed to use it right through your cycle and for a minimum of 3 cycles.
Presuming you want it for fertility then you only use it until you O. Well, at least that's my understanding anyway!
I have a fairly regular cycle but it brought my O date forward by one day this month, but I'm not sure if I will continue with it because it caused me almost unbearable O pain as well.
It seems to get a really mixed response - I hope you have a great experience like Molly and are posting in the BFP section soon
