I'm not sure where you got 8 hours, not that I know differently. I do know that how long something you've ingested takes is affected by a whole slew of different factors, from weight, age, overall health, how fast you ingested it, how hydrated you are, and of course how much (and how concentrated, etc.) you took.
Testing I think is pointless, and is probably more of a hesitation mark thing than anything else. Were it me, I either go or I don't, because anything in between is likely to land me where I don't want to go. That, and I hate barfing…