Hi - I believe some side effects build up gradually (weight gain), while others can happen quite quickly (hot flashes, possible depression). Of course, if you're talking the dangerous side effects (endometrial cancer, stroke), they'd take longer. I think everyone's experience with side effects is different; but my impression is, for most women, if you get them, you keep them the whole time. Not to say they're not treatable - depression certainly is. And some women have luck with vitamin E or antidepressants for hot flashes. Deal with them as they occur by talking with your doctor, and seeing what (s)he recommends. Best of luck to you - PJH