Good topic...wish I actually timed the release but unfortunately have always just eyeballed it and watched for efficient mechanics and what the pop time was. Not very scientific I admit and I don't really have anything to hang my hat on when I work with someone on improving his release...basically watching velocity of the thrown ball and the pop time and letting that guide to where the problem might be if the time isn't good.
CatchingCoach has this covered pretty on how he does it on his website. What he says makes sense about timing the release.
A good,quick release has saved alot of catchers over the years that don't have cannon arms.