A friend who runs a public aquatics exhibit quarantines each fish for 6 months.
I try to go at least 3 months.
Many diseases do not become apparent in a short time, some like ick are usually almost immediate, but it's fairly easy to work with.
Its tha bacterial diseases that aren't so obvious, and not as easily dealt with.
I had some Mayaheros beanii that arrived with colomnaris, it took almost 3 months for it to become obvious as to why it was , and they all died at about the 2 to 3 month mark, if I had put them in the main tank, I believe every fish in the tank would have died.