When to replace tires on 2008 F-450 dually? Age? Tread wear?

Just went shooping for 20 inch tires for my F250. Since I didn't have a dually I did not want to push them. I checked Discount Tire, and Ford. For the past couple of years Ford has had the best prices. I ended up with BF Goodrich, and really like the look of them. With your Dually and plenty of tread I think I would run at least one more year and maybe 2. Watch for crack in the sidewalls. Definitely pull them at that point