6 months shouldn't be to far off. It depends whether it is a wild, captive bred, captive raised one. I know that the first regals that were bred in captivity were roughly an inch at two months but they didn't have the strong blue colour or any yellow yet. I think it's about this size that they begin to settle out of the plankton on to the reef.
Sometimes today fish are harvested before they settle on the reef and then acclimated to a captive environment for a few months before being sold, so 6 months is a good estimation.
If given enough space and care it might be with you for another 20 years or so.