Got that answer from Tortius a long time ago.. He says 10 days.. I will take his word on it..
I've gone with just seven with no ill effects but wouldn't suggest it to anyone not wanting possible problems.
The process has alot of varibles.. I would tend to think less, the bactiria in question reproduse twice fold in a twenty four hour period (FAMA cierca late 90's) so the sutable surfase area (the new filters media) and the active colony you have now makes a diffence wich is dependent on current bio-load (that comes down to how much food you have been feeding)
All in all.. just putting it on there and giving it at least 10 days should get the job done (In the land of perfect it would take 24 hours, LOL and I know I dont live there
And running them both on your old tank is the smart thing to do.. like TomK2 said