Cedar makes excellent line posts, and will last quite a while. My parents have a fence on the east line of their property with some 40-year-old cedar posts in the wire. However, it should be noted that some of those posts have rotted in the ground and most of the fence is being held up by steel T-posts, driven deep into the hard, unforgiving, rocky, dry West Texas earth by yours truly.
My dad uses slender cedar posts for "staves," as he calls them, posts that are put between steel T-posts for added support to the fence. For example, if your fence has a steel T-post every 8 feet, you could put in a cedar stave at 4 feet. Now, some may say that's a wall, not a fence, but Daddy liked to build them high and tight, and I guess I inherited that outlook.
I don't think I would use cedar for corner posts, but then again, I am partial to re-purposed telephone poles. I have seen plenty of cedar corners and H-frames around West Texas and in Central Texas, though, so I know some people use them. I don't have any idea about drying time.
Anyway, there's my 2-cents worth.