I have been on synthroid for over a year now, but I seem to remember feeling a difference after a couple of weeks. One day, I just wasn't as tired as I had been. I'm still tired all the time, but not as much.
I did have an incident with my dr (getting a new one) where my Rx wasn't refilled in time and I was without for 5 days. I lost nothing that week, but the next week I lost 3lbs which was my highest amount yet. Hang in there!