I didn't get a BFP with a HPT at all, only with a BT, as did Mel (Melbo) I think, but if you must HPT (and if you're anything like me, you will!) I reckon definitely not any earlier than 11 post transfer with a day 2 transfer, or 9 days post transfer for a blast (which mind you would make you 14dpo anyway!) Blasts are transferred the day they would normally implant, so it's been known to display lower HCG levels at 14dpo than day two transfers.

Did I make sense?