There are a number of kinds of cedar. Eastern red cedar, found from east of Colorado and into much of New England is actually a juniper, a member of the cypress family. True cedars are members of the pine family.

Junipers are tough, resilient trees that make excellent fence posts, either whole or split. You do not need to treat or debark the posts. As to how long they will last, that depends on how moist the soil conditions are and the amount of freezing and thawing the posts will endure over their lifetime. In the Midwest, Eastern red cedar fence posts have been found to last for decades.

Here is the life expectancy (in years) of some wood fence posts, from the Virginia Cooperative Extension:

Black locust, 20 to 25

Hickory, 5 to 7

Honeylocust, 3 to 7

Osage orange, 20 to 25

Red cedar, 15 to 20

Red oak, 5  

Southern pine, 3 to 7

Sweetgum, 3 to 6

White oak, 10

Yellow-poplar (Tulip tree), 3 to 7