I can always count on good questions from bros like you, 8and20. You are absolutely right. Most research quotes 6 months as the time needed for visibly improved BPH (as measured by MRI or Trans-Rectal UltraSound). Of course, we both know that doesn't necessarily mean sx improvement. Most MDs I know use it for those pts who won't or can't under surgical TURP.
And yes, it will generally interfere with cycles, not to mention libido!
Here's a good article on the subject:
Prostate Tissue Composition and Response to Finasteride in Men With Symptomatic Benign Prostatic Hyperplasia. The Journal of Urology, Volume 157, Issue 6, Pages 2171-2178