Clearly there are many factors involved in pole life expectancy. Where I grew up here in Arkansas, (a property my mom still owns but doesn't live at) there is a pole hay barn built by my grandpa in the mid to late 1950's. We get some pretty serious weather here from tornadoes, high winds, flash flood thunderstorms, and rarely 1- 2 feet of snow. That pole barn has stood up to all of these conditions for 50 - 55 years or so. I haven't been out there in a couple years and I didn't have quite the same building/construction enthusiasm as I do now but I do remember the basic structure. It's a decent size barn, maybe 35 x 30 or so. It originally had a tin roof and siding but it was replaced 7 or 8 years ago with the the regular corrugated steel panels. It has a dirt floor but my mom put gravel over part of it a several years ago.
The poles are what people here call cedar or red cedar, though it is actually a species of juniper. It's very common locally and was used in many of the old farm structures. I have no idea if they were treated by my grandpa but those poles have been in the ground for 50 - 55 years with the bark still on them and showed no signs of rotting whatsoever a couple years ago when I was there. There is a 2 x 8 or 2 x 10 treated drip board all around the base and the eves are around 2 feet if I remember correctly. I believe it's very slightly higher than the surrounding ground.
I have never dug up a post but I'm pretty sure it's just stuck in the ground with no gravel or anything whatsoever around it. The poles have probably never been subjected to direct splashing from water but the ground does get very saturated around here certain times of the year.
Those red cedar / juniper posts seem to be pretty awesome in the right conditions. I am using a lot of them from the property I purchased in 2012 and plan on using them in my hybrid pole / psp / earthbag structure. As Paul mentioned, it's all about diverting the water around your structure so that it wants to go somewhere other than towards your posts.