It really depends upon you. But Tracey Broderick (Slimmer of the year 2007) lost 11st 4lbs in 8 months which was over half her weight!
I am not saying that everyone does that, as Porgeous says Cambridge say it is about a stone a month after the first month, but stick at it & always think of your long term goal not what this week has dealt you out. Some weeks you may have a low loss & the following week it could be a great loss. If you CDC does not measure you each week see if you can get someone else to. I always find the weeks my customers have low (disappointing to them) losses they usually lose lots of cm (I always do cm & measure lots of different points!)
Cambridge sell a 2m tape measure, see if your CDC will order one for you. Having said that I have not used mine yet. The largest lady I had was before I had my Cambridge tape & she was a few cm bigger than 150cm at her largest point.
I always measure the smallest point around your tummy (usually a few cm below your boobs), the largest point (about!) (in line with your belly button), around the hips (in line with your bits!)...as you can see I choose measureable points that I can do on fully clothed people & get the same point each week. I also do chest, both thighs & both upper arms.
Seeing your measurements come down each week is a great incentive.