A Fantasy Author’s Search History: How long does a wooden bridge last?

I had needed to know how long different kinds of bridges last when I was deciding what material to make the bridge in Lythannen out of. Wooden bridges only last about 20 years or more, depending on the type of wood used, how it’s treated, and the maintenance. I did find that they had ways to treat wood in medieval times. And covered bridges can last up to 100 years.