I've never had Dunlops, but based on past experience I can echo a couple of sentiments that have been expressed here.
If you're happy with the tires you've got and they're working for you, then don't worry about it.
But, as soon as you begin to lose confidence in them in certain driving conditions (like rain or snow), it's time to change. Don't wait for them to wear out - change them right away, or at least before the next rainy/snowy season. If you wait, you won't get as much enjoyment out of the truck as you should, and it's silly to get frustrated with a vehicle that costs tens of thousands of dollars over a $600 set of tires.
On a couple of previous cars I've owned, the OEM tires were OK for the first couple of years, but it was amazing how much the ride and handling improved after switching to a better set of tires. It was like having a new car!