how long can a mystery snail stay floating?

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I have a golden mystery snail that has been floating for a couple of
days now. I have read that as long as the trapdoor is shut tight, the
snail is not dead but how long can they stay like this without coming
out to eat?

This snail has been fairly immobile for the past week or so, but it
would come a little bit out of its shell. Finally it shut itself up
tight and stayed on the bottom of the tank. It didn't move that I
could tell. I was afraid it was dead or dying so took it out and put
it in a 2 gallon holding tank with a little hornwort and an algae
wafer. After about half a day, it floated to the top and has been
floating the last couple days. I change the water every day and put in
half an algae wafer. The trapdoor is shut tight. I've also read that a
dead snail has a terrible stink but this one just has a kind of fishy
odor.

I also have a blue mystery snail and a black one and they are active
and don't just sit in one place for all that long or float for more
than a short time. How long should I give this snail before deciding
it is surely dead? Is it true that as long as the trapdoor is tightly
closed, it is still alive?
-yngver