I've been told by numerous docs (ob-gyns and regular GPs) that it is safe to take it back-to-back, although they differ on how long is ok. I've been told everything from 4 months to a year.
I know it seems strange to not get a period, but it seems to be a fairly commonplace thing now, it's a very common treatment for endometriosis, and a lot of women do it just for conveinance.
That being said, I don't think there have been any long-term studies on the effects of continually skipping your period...that does worry me sometimes. But, I can't stand the pain once a month and it's supposed to keep the endometriosis at bay, so the benefits outweight the risks for me