I don't know - and I'm not sure there is an average first loss. Think about how different we all are at the start and how much (or little) we all have to lose. F'rinstance, I lost 13.5lbs in my first week but then I have a massive amount to lose in total and the changes I made to my eating in that week were startling
I do know that the losses for many people can be larger at first hen gradually reduce to a more sustainable ongoing total. Every loss counts!