I don't take Met anymore (I no longer need it, as my IR is in "remission"! YAY!) but when I was taking it, I found it took a month to really notice that I wasn't, like, ravenous all the time. I did suffer pretty badly from stomach issues the first month, though, which is probably why it took me so long to notice. *L*
It didn't make any difference to my periods, though. When I was originally diagnosed with PCOS in November of 2007, I was put on progesterone-only BCPs, and that regulated my cycle pretty much immediately. When I started the Met, my doctor pulled me off the BCPs to see if my period would gain regularity with the Met.
Didn't work. In the year that I was on Met and off the BCPs, I had exactly two periods. And my doctor kept me off the BCPs for a while even after I came off the Met, but still nothing. So in my case, the IR obviously was not what was causing the amenorrhea; it's probably due solely to my extremely low level of progesterone. It seems as though, without some synthetic form of progesterone, my body simply does not produce enough on its own to allow me to have a period. It sucks, but...eh. Could be worse.
Anyway, yeah. Metformin does work for many, many people, but in some cases, it doesn't. Hopefully it will work for you!