DS isn't on 6mp, bit it's sister drug, Azathioprine, which I've read tends to have more side effects than 6mp. He didn't experience any notable side effects. A weird joint pain here or there that lasted a hour, maybe. Besides that, nothing. Then, much to our surprise, his labs showed he was at therapeutic levels already at only 5 weeks. We are at seven weeks now and he feels great. For most people it usually take 2-6 months.
Just be sure to get regular lab work done to monitor her liver, etc. Your GI will give you a schedule to follow. Weekly, then bi-weekly, then monthly. Very important. Also, if she does have side effects, you can stop this drug instantly (per your GI) and the side effects will clear up quickly.
Good luck!