It will take 4-6 weeks for zoloft to take full effect. I wouldn't worry about xanax withdrawal. People tend to develop a resistance to it within the first month which is why they are not prescribed long term. At the most you will feel slightly depressed for a day or two max. Hang in there!
i too am on 50mg of zoloft. it helped alot with my anxiety. its worth it. hang in there. best wishes.
I go through anxiety too and found zanax is instant so it helps when I have panic attacks / when I'm driving . im prescribed 2mg bars equaling 6-8 mg a day and that's a low dosage compared to what I've been taking you won't have with drawls from the amount of xanax your on. I have never switched anxiety meds but hope Zoloft works for you.