When to test when you have a long LP?

[TripleRock]

Do you have to wait until AF is due or is 14 dpo
long enough?

TIA TTC gurus

I think you'd probably get a BFP if pg at 14 dpo as long as you're sure of the O date. Obviously the 'sensible' thing to so is wait for AF and save money, but I would test at 13 or 14 dpo ;)