At what mileage should you have to replace the upper and lower ball joints?

Unfortunately, Driving conditions have a lot to do with the longevity of ball joints, so mileage is not the only factor, but from what I have seen, I would not expect to replace them at less than 80,000 miles for that vehicle, there is one other manufacturer, ( not Ford) who is experiencing ball joint failure at 35,000 miles, due to the material that used for protective boot, ( like a bio-degradable plastic ); which crumbles and leaves the ball joint exposed to the elements. ( Just my opinion, from what we see here at our facility )