It's usually expected to last up to 25 years.
I've always used treated timber and given the post ends 2 or 3 coats where they will be in contact with the soil & left them soaking in an old oven tin full of the preservative overnight to soak up more & increase life expectancy.

Timber rots when in contact with the ground and also rots in concrete because the moisture can't drain away.
The latest idea is to dig the hole deeper then put gravel at the bottom of the hole, put the post in and add a bit more gravel so the end of the post can drain then fill to just above soil level with concrete and slope the concrete so rain runs away instead of into the post.

You would be better off not burrying the wood in the ground but build some brick piers and rest the structure on them. It will need some restraining straps, but this is the way to go if you want it to last any length of time.