There is no magic number of miles where they go bad. DOHC 5.4L's were used in Navigators, and now of course in some of Fords super cars. Even though the original DOHC 5.4L's had 300 hp their torque curves weren't very good compared to the 260hp PI 5.4L's from everything I have seen.

 

I hope not I got like 120,000. kinda maintained. Just did plugs not sure if 1st time the old looked ok. Reg oil changes,fuel filter, air filter, coolent change what else? I hope it lasts past 135,000 The new plugs made a big difference in performance.