I was searching for "asteroidea" (biological term for sea stars) on google scholar, and found the term "desiccation" alongside it. So I did a search for exactly that, but found no exact time spans. However, it seems like some sea star species have to survive periods of dryness during tides, so I'd say 12 hours should be about a maximum. Just a wild guess, though. It apparently matters if there is sun or not as heat will dry them out quickly.
Anyway, I learned a lot about sea stars in the process
Humidity level should matter, too.
Anyone know how long a starfish can survive on land, and if it matters especially whether there is a hot sun.
I promise this will turn out to be (at least somewhat) important and Less-Wrong relevant. Googling for the answer gives me a bunch of "wiki-answers" type sites that I don't trust and news articles that don't give me the specifics. If anyone either has marine-biologist cred, or better google-fu than me, I'd appreciate it.