Once you go off of HRT, you lose bone and benefit and will potentially experience menopause transition symptoms once again. If you go off of HRT, to avoid bone loss you'll likely have to be on something so it's really a choice of osteoporosis drugs or HRT and sometimes both. As far as testing, an annual uterine ultrasound is a good idea as well as mammography and/or breast ultrasound. I believe I'll be on HRT for the rest of my life barring any contraindications or problems that present.

I've been thinking about this for awhile. One of my obgyns who uses hrt herself did tell me that the use will be for the rest of life unless problem(s) arises. Like @teb pointed out, there are periodic tests/exams and labs need to be done to assess ongoing risks vs benefits. As far as the reoccurence of menopause symptoms after cessation of hrt, my hunch is it will be inevitable, but the magnitude of which is questionable. The estrogen receptors are largely downregulated 10-15 years post menopause, even they get upregulated again by HRT, the biological effects of estrogen won't be as robust as when we were young. If you didn't have troublesome vasomotor symptoms last time, I think its unlikely you'd have worse experiences the second time around. But, all these thoughts are just thoughts. In the event when you are off hrt, you'd definitely follow up with a bisphosphonate or better yet, an anabolic.