Complex sentence: The service was 26 years (as opposed to the 20 to 25 years for a legionary), from the 3rd century it was 28 years, occasionally it was even longer.
Split into simpler sentences: The service was 26 years (as opposed to the 20 to 25 years for a legionary). From the 3rd century it was 28 years, although occasionally it was even longer.