I have now been on anti-depressants for almost five decades. In that time, I’ve used just about everything out there, and transitioned from one to the next. Unfortunately, the honest answer is that how long they take to kick in varies widely based on the specific medication. I’m sure the patient’s condition can also change it by quite bit. My average has been around a month to get a useful effect, but my minimum is probably around a week and my maximum more like three months.