Purging bright tanks
The process for purging a bright tank to be transferred into as I was taught is to blow CO2 in through the bottom with the valve cocked until you no longer smell air coming out of the blowoff arm. Instead you are knocked over by the brutal aroma of CO2. Close off blowoff and pressurize to 10psi.
Sounds reasonable. If you wait a bit however it seems like you can get "air" coming out of the arm if you open it.
How long does it take to purge every last bit of O2 out of the tank?
Does it really matter that there may be a bit of O2 left in the top of the tank knowing that CO2 is heavier than air and is blanketing the incoming beer while forcing the residual small portion of O2 out?
When is the tank purged enough?
It all depends. For example if you have a BBT for brewpub use and it will empty in a few days, you don't even need to purge, just pressurize with CO2 and enter beer from the bottom. Did that procedure for years with award winning unfiltered beer. The beer in the tanks typically only lasted a few days so really no need to purge. If the beer is for longer periods I do recommend a purge. What I do presently is to pressurize with CO2, then vent (the air/CO2 mix) then pressurize again. Would be more cost effective if you have a Nitrogen generator and a CO2/N2 mix.
As long as you are filling from the bottom and the beer is entering the tank 'quietly', ie. not looking like a fountain!, then you shouldn't have any significant O2 pick-up.