The average is a stone a month for women, stone and a half for men. but it's just really variable from person to person.
Best thing is to not compare your losses to anyone else. I've seen first week weigh ins go allt he way up to 18lb, but also you may lose 3lb or even less. It's all different.
But good luck - and that first month especially you should definitely expect to lose a stone i'd say.