A minimum of three days if you want to be cautious. A week is better and two weeks is ideal though after a week, usually you'll find out whether the fish have something or not. The extra week is just an extra precaution in case there's something slow to reveal itself.
I'll have my newly aquired Khulis in quarantine for at least a week though if they show signs of good health, I might let them out early, or if they look sickly or stressed still, another week to be safe.