It varies. In most hobbyists tanks they may survive about 6 months to about a year before dying of starvation (without obvious signs of trouble - they just start disintegrating). In certain tanks, or certain individual stars, they may do OK. It comes down to surface area of sand, the health of the sand bed, and whether or not the star will consume other foods. Most do not, and most are not in large enough tanks to sustain them for more than a year. I would say few survive past 18 months in captivity...their life span in the wild is relatively unknown...but certainly FAR longer than in captivity. In fact, there is discussion in the echinoderm world (including seastars) as to whether they ever really die of "old age."