Coral are actaully a colony of individual polyps. Each polyp is an animal in itself that contributes to the colony. So you can have polyps that die or even sections of a colony that die with out destroying the entire coral. As long as the environmental conditions are acceptable for growth of the polyps i would think a coral colony can live indefinetly. There are SPS
corals in the wild that measure better then 10 feet across. As cmor said the coral will outgrow your tank and will need to be trimmed back as it continutes to grow.