It varies, and it's difficult to give an exact answer. A character which is fifty years or older is considered "old" and might drop dead any day, but generally you will see rulers living into their sixties or seventies, sometimes even into their eighties ... and very rarely into their nineties. But it depends on the health of the character, and there are several traits that affect this. Base health is 5.
Strong: +2 health
Martial education: +0.5 health
Weak: -1 health
Stressed: -1 health
Wounded: -1 health
Imprisoned: -1 health
There are several illnesses that affect your health stat negatively – some of these can be cured, some are permanent.
A healthy character is less likely to become ill, and is also more likely to be cured when they do fall ill. They are also more likely to live longer lives.
Furthermore, there are some other factors, which determines the likelyhood of a character being killed in battle.
Leper: 2.0x more likely to be injured, killed or imprisoned in battle
Possessed: 1.5x more likely to be injured, killed or imprisoned in battle
Drunkard: 1.1x more likely to be injured, killed or imprisoned in battle
Craven: 0.5x less likely to be injured, killed or imprisoned in battle
See full list of traits, and their effecs on health and battle events.
With the new Way of Life DLC, there are also duels. Your character might be a good or bad duelist, depending on his "personal combat score". This is affected by several traits.
Finally, your character may become a target for assassination or imprisonment (if you are a vassal). The likelyhood of a successful assassination/imprisonment against you, depends on your state intrigue, or whether or not you are in hiding.
Long story short. For a long, healthy life, make sure you have good genetical traits to begin with (e.g. "Strong"), and educate him/her in the martial way, preferably with someone with good health traits. Never go on hunts, or lead battles, or fight duels. Make sure you have a good state intrigue, and be good friends with your spymaster and vassals.