It varies quite a lot but I would trow a guess at it: 2-3 weeks and we will see a surge in P800 related sightings and 3rd kind encounters.
Note that I do not expect to see it in any significant numbers before late November.
For me, the BT certification means, in one hand, that from now on all the units displayed in public (or lent to reviewers) will no longer have the status of prototipes, and on the other hand, that mass production is about (or has already) begun... (call it wishfull thinking).
Lets look to T68m whene it gets certification and remind where was in market.
I can find that I bought my t68m about 6 weeks after geting certificate and it was some weeks after oficial release.