I keep them in QT for observation. Not so much for a problem they have, but more for a problem they might carry in.
I inspect their shells closely, looking for Aiptasia, hair algae, or any other undesireable hitchikers.
I keep them in QT for 6 weeks to make sure they will live. Many so-called tropical snails are coming through the system that are really cold-water specimens that die pretty quickly in tropical water temperatures.
Also, the 6 weeks is a pretty good guarantee that they didn't bring any obligate fish parasites with them. If I think they may have come from infected water or an unknown aquarium, I let them stay there for 8 weeks minimum. No rush.