I'm sure there is a poll on this somewhere, will have to do some searching. I found out at 5.5weeks with Marisa as she was a surprise package, but Elijah I tested at 9DPO 8-[ and got a faint positive in a Discover Early test.

My sister on the other hand, is almost four weeks and can't get a BFP by test so has had a blood test, despite having very strong pregnancy symptoms - so just goes to show the body works in mysterious ways!