Flour typically lasts for 1 to 2 years in the bags that the flour comes in.
For the best taste and freshness you should use the flour within 1 year of purchase or it can start clumping up and change colors.
The flour is still safe to use after a few years but it can begin to get hard especially if it's in an area that has high humidity or moisture.
Keep the flour inside sealed containers and store the flour in a non humid place and the flour should last longer than a couple years.
Although I don't recommend keeping flour for 20 years my grandma when she died has flour that was 20 years old in a sealed container and I took it home and used it.
It wasn't as fresh but it was okay to eat.
But it's always recommended to use flour up within 1 to 2 years of purchase for making the best baked goods or best food with the flour.