Assuming you are asking how many miles the truck will run before the engine dies, I would say with proper maintenance, you could get up to 300,000 miles out of it. The 5.4L Triton engine is a reliable engine, but like any other mechanical component, it will eventually wear out. Of course, there are many other factors that can affect the longevity of an engine, such as how well it was manufactured, how it has been maintained and operated throughout its life, and even luck.
If you take good care of your 5.4L F-150 and change the oil regularly (at least every 5,000 miles), you can expect to get hundreds of thousands of miles out of it before needing a major overhaul or replacement.