Have you quarantined any of your other fish?
If the answer is no, then you already have ich in your system. Anybody that tells you otherwise is uneducated and shouldn't give advice. In that case I would say observational quarantine only for 2-3 l weeks. That'll ensure you aren't getting velvet or brook. Those are super aggressive and will show in a week or so.
If you HAVE quarantined your entire system (including snails, rocks, corals, etc) and every drop of water entered into your tank is either from a fully medicated fish quarantine or a 76+ day fishless quarantine, then I'd say definitely quarantine as you did the others..
My guess is that it's the first one.