How long does it take fibroid to shrink after menopause?

Variable. Reduction in size for fibroids is desirable although that is not the basis for treatment. Symptoms are why someone with fibroids is treated. The most common symptoms with fibroids are heavy bleeding, pelvic pain, and increased urinary frequency. If you're post-menopausal, bleeding no longer an issue. Other 2 should start to go away over time; if not you could consider uterine fibroid embolization.