What do you mean with "righting" do you mean for them to turn themselves over again once they have been turned over? Its an interesting question, either you'll be lucky to find that another researcher has already investigated this or you'll be a ground breaker. Good luck.
This reference doesn't look into your specific question however it is a good paper on righting behavior in stars in general, including the time it take. Good luck!
Kleitman N. 1941. The Effect of Temperature on the Righting of Echinoderms. Biol. Bull. 80:292.