when i upgraded from a 45 to a 90 gal
... i also upgraded from a HOB
filter to the Eheim 2217.... i placed my used filter media into a nylon bag and stashed it in the back of the tank behind some drift wood for almost a month....
took it out last night... hope it was long enough! then again all my substrate and decor came with it too, so i think i should be ok.
short answer: no less then two weeks IMO