There's no such thing I'm afraid. A healthy weekly loss is 1lb to 2lb. Some people loss more, particularly if they have a lot to loss but it will eventually even out to around this. And some people lose less. But we are all heading in the same direction and even 1lb a week adds up to nearly 4 stone over the course of a year.
I'm going to answer this with the statement of "Average - what does average mean?" - just replace "average" with "typical".
The plan is designed to give a loss of 1 to 2lb a week. Wether you get more, or less, is down to so many factors - how well you stick to plan, how much activity, how much you have to loose. I've been a member now since December 2008. My losses are anything but typical though.
My first week - I lost a stone. (remember though that I started with over 27.5st to loose). I lost 13.5st by end of April 2010, and have lost 2st4 since then. (but have spent a lot fo the time since June last year trying to maintain). If I have a loss these days - it averages about 1lb a week. At times though I was regularly pulling 6 or 7lb a week at the start of the journey and a stone a month for the first 8 months.
That said - I know people who have taken 2yrs or more to loose 3stone. It's all so variable - that really - you can't take any average or typical. Just what the plan is designed to give.