How many years was it between Paul's conversion and martyrdom?

It should be noted that Paul himself, in Galatians, mentions that he returned to Jerusalem again 14 years after his conversion, but there is no further information on the events of these years. We also know that, following this final visit to Jerusalem, Paul was held prisoner for two years, until his case was reopened in AD 59. After this, he used his position as a Roman citizen to appeal to Caesar, and he was then escorted to Rome in AD 60, where he awaited trial for another two years. This would mean a minimum of 16 years (and an unknown maximum) for the time between Paul's conversion and his martyrdom - if indeed he was martyred.