How long should you stay on Effexor XR.

I've been on Effexor XR 75mg for so long I cannot quess when I started. Is this for short term use or extended useage? Due to financial issues I have not taken any in about 3 weeks. Now I feel like I'm coming apart inside. I can not sleep at night because I'm so figity. Every little thing that goes wrong I feel like the world is coming to an end. I was under the impression this was not a habit forming drug. Help Please