I'm not sure it ever does. I've been on it for probably close to a year. The only thing I'll say for it is that I'm not curled up in a corner sobbing anymore, but I still have anxiety, I still don't feel good. I'm going to the doctor tomorrow (for another issue) and I'm going to see about getting off it. I've read that it can cause short term memory loss, and I'm seeing some signs of that.
Are you taking Wellbutrin XL or SR? And what dosage? I would give it another two weeks. If you aren't feeling something in 4 weeks then call your doctor. I think folks generally feel an effect from Wellbutrin a little more quickly than with SSRIs or SNRIs.