A quarantine period is typically 6-8 weeks. If one was to start a new tank and add a fish or two each week for 2-3 weeks, I'd still consider the new tank a quarantine tank. After that, the gamble and risk goes up.
Having written all that, admittedly many hobbyists roll the dice and will add new fish to their existing aquarium and perhaps 80% of the time all is well. A lot depends on the fish and the dealer. Sometimes I think it' can be safer to deal with a Mom and Pop local store than a big chain. It also doesn't hurt to ask questions like 'are these fish quarantined?', 'have they been treated with meds?', 'what's been your customer satisfaction with fish?' ...