I did my initial from the pour to Primary, then waited 72 hrs as I saw the bubbling stop, just to check how far along it was when it stopped bubbling.
Reason I figured was that the yeast had probably generated enough alcohol to prevent any infection from breaking the seal on the Air Lock. Re-installed air lock when done taking sample and let it sit. 8 more days to go then to secondary. I'll take my next reading then.
Cheers and congrats on your first brewing! I'm only a few days ahead of you.