I agree with most people said here, especially that everyone is different. That couldn't be more true! I did weight watchers with a friend of mine, and we both had the same amount to lose. For some reason, her body didn't really react negatively to anything! As long as she followed plan and worked out, she could pretty much predict what she was gonna lose. Me on the other hand, completely different story! I literally gained every time the week before my period. No matter what I did, how much I exercised, it was like clock work that I would gain that week. Also, on some weeks when I did extra exercise I would gain. Which doesn't seem to make sense. Then I decided to start taking my measurements in addition to weighing. This way, when I was turning my muscle into fat at the gym, the scale may have went up 1 lbs, but my waist was also down 1 inch. And to me, how I look, feel, and fit into my clothes is more important that what the scale says.
I would try not to focus so much on "how much will I lose by X date". Only because if life gets in the way (as truthfully it often times does) you can really discourage yourself. Maybe buy a pair of jeans that you want to be wearing at your goal. Hang them up and try them on every few weeks. Maybe you only lost .5 lbs one week, but if those jeans fit a little bit better, isn't that whats important?