It's really going to depend on the wood, the ground and your climate. for example here a 10" telegraph pole will last 20 years when used as a fence post, and that's after a life as a telegraph pole. but a 4" preasure treated softwood fence post will only manage 10-15 years. We have very wet ground with 4 months of constant freezing/defrosting action.
we ourselves also die;
but one thing never dies
the fame of him who has earned it
I have three kinds of treated fence- creosote, arsenic copper and whatever is done now. The creosoted are at least 40 years old and fine, the copper arsenic are now starting to show rot at 10 years and the new ones barely make it 5.
The quality of the wood has gone down too so it's hard to say which is the source of the problem. I have 100 year old plus redwood logs that were unburied when I built my house and they show no rot at all.
So it depends.
Resentment is like drinking poison and then hoping it will kill your enemies.’ Nelson Mandela
We put in a wood deck, large, about 25 years ago. The lattice broke down a few years ago and I tore the lattice out. I'm thinking of replacing it but if the deck is going to fall apart soon, should I really invest in it? I'm seeing rotted stairs (one inch thick deck boards) in the past couple years, so slowly tearing those out and they need to be replaced too. I haven't noticed any instability in the posts, yet.
When the posts were purchased they had tacked on warranty information that these posts would last 30 years. I'm pretty sure we are approaching the end of that.
The cement issue. Some say that setting posts in cement is a problem as they break down worse with cement. Some say it is a good idea. I'm pretty sure we put cement powder in the hole, then added water, then set the post in it. There are 12 posts. I've seen some information on the internet that these posts can be replaced when they need to be replaced. We dug down 3 feet when putting them in the ground and have no heaving through the seasons.