Dropsy is usually bloat and/or the pineconing. Otherwise it sounds like a tumor. Can you get a picture of it (including bird's eye view)?
As for the snail, my own personal rule for the scaleless is 6-8 weeks. But this is ONLY my own personal idea. I prefer to quarantine them twice as long because they don't tend to show things in 2 weeks, like most fish (in most cases) do.
HOWEVER, if you do not suspect a bacteria, fungus or parasite to be the culprit and it is just the tumor that has caused dropsy then I would still do a quarantine, but wouldn't have it for as long as I mentioned. 2-4 weeks I'd say, if you are absolutely certain. If you are unsure, it never hurts to go longer.