The 3rd Century Crisis saw about 50 men claim to be the emperor over a 50 year period, only half of which were recognized as such by the Senate. Nearly all of these men died violently, most commonly at the hands of their own soldiers. Most could measure their reigns in months, a few in days.
Gallienus held power for a remarkable 15 years, far more than any emperor during the Crisis. He was eventually murdered.