Everyone differs as it depends of your levels, and the type of test as they measure different amounts.

From memory, a thread i posted in here about when did you get your BFP, the ladies ranged quite a bit in how many DPO they were when they got a BFP. I think the earliest was 8dpo up to around 16-18dpo.

If you know your usual LP length, then that will give you the best idea when to test.