I normally wait until a couple weeks before I know I am going to be purchasing fish, and then drop a Hydro sponge filter in one of my already established tanks to seed it with bacteria.
This way, I don't need to run it all the time in main tank(s) if it is aesthetically unappealing.
I run one tank however for plecos, and it usually alway's has a sponge filter going in it so moving it to quarantine tank is easy and helps with my sometimes compulsive purchases (always have a seeded filter)
Four weeks is minimum for me, otherwise you may as well not quarantine at all. As already mentioned ,,some pathogen's ,won't present themselves for a couple three weeks.