I'll look at non hereditary absolute rulers first, which are similar to monarchies. If you look up Dictator, you'll find examples of absolute rulers who had set terms. Roman Dicatators being the original example as well as Giuseppe Garibaldi.
Many states have or have had emergency power laws that effectively result in a dictator for the duration of an emergency (although sometimes this doesn't go so well, Article 48)
Protectors spring to mind as well. So do puppet rulers, colonial governors, Proconsuls and others who ruled like kings but bowed a knee to an external authority.
As for monarchies, recently with Queen Beatrice's abdication we've been reminded of Dutch tradition, where an aged monarch is expected to abdicate. But did you know it was also expected in Medieval Japan, where it was practice for emperors to step aside so the heir could ascend relatively young. In addition, before Meiji Restoration it was common for empresses (women) to rule for the minimum time until a male descendant was old enough to rule. Women leaders acting as a temporary "stop-gap".
None of these aren't hard and fast term limits, but they are clearly informal limits on length of rule.