I quarantine for a month after the fish are healthy. If they're healthy from the start then it's a month, if the get ich or something 2 weeks in, then it's a month from when they're cured of whatever they had. So the total time could be 2 months or more. Some may think this is longer than it needs to be, but it's worked for me and there's nothing worse than killing fish you've had for years because those fish you just bought looked "so healty"!
Edit:
I do break the rules sometimes if I'm buying fish someone I know very well has bred. If I know they're as careful as I am then I'm less concerned.