I've quarantined twice since I owned chickens. The first one, Hawkeye, was kept in a dog kennel, an extra large wire one, in the bsmt for five weeks. We have two of the kennels and I put the other one up on the deck, which is about 6 ft above the ground. On sunny days, a bit into the quarantine period, I'd bring him out to get some sunshine. The girls were nowhere near him during that time.
The second time, I was given an Ameraucana as a gift from my very trusted friend, Kate (helmstead here on BYC). She was 8 weeks old and by that time, we had built the little Firetower coop and covered pen, so we kept her in there for a month. She came from eggs from a breeder and Kate has never had any health issues with her flock, either, so though I was pretty sure of her perfect health, I quarantined her anyway.
IMO, though quarantine is not foolproof, it's the least we can do. It should be 30 days or longer, preferably six weeks, really.