I think it depends on how early you want to test, or if there's any significant importance to finding out as early as possible. Some prefer to wait until their period is due or wait to see if they miss their period, so if things vary for you and you aren't bothered about finding out early then you could leave it until the latest your period would usually show up then test if it doesn't. Or, you could go by dpo instead, if you want to test as early as possible. In that case, most don't see a positive until after 7dpo, earliest. Usually it's incredibly faint that early as well, which is another reason some prefer to wait longer so there isn't any confusion. I got a light but definite positive on a FRER at about 10dpo with our son