melissa - I have seen conflicting accounts of when you should know if it is going to work. Anywhere from one week to over a month! Has your dr prescribed it to many patients? If so, you could ask her what she feels the response time is. Wish I could help more - I lasted three weeks on it before side effects made me stop. I do think it gave me more energy, but I didn't feel any difference in the pain levels.
Also, now that I have stopped my pain is worse than before I started! I hope it is a short term effect from stopping it quickly.
I hope it gives you pain some relief soon!