Hmm it really does depend when implantation occurs. As soon as implantation happens you start producing the hormone that hpt's pick up. I think the earliest most people have had a bfp is at 10dpo.
With Charlie I had it at 11dpo and that was with a first response one.
I don't think you can generalise so much as different pregnancies have different hormone levels and therefore different results!
Good luck x