Cannonicaly its 7days ... for regular tadpole.
(If im not wrong laugh )

But ingame there is no time limmit, or amount of long rests that would turn you into Mind Flayer ...
Since you can withness metamorphosis on the ship, started by "pulling the lever" ... i believe its safe to presume that this is the only way this alterned tadpole can turn you.

I still dont understand why cant we change Race for our hirelings. frown
Lets us play Githyanki as racist as they trully are! frown