Ok, so I called my service guy at my dealership. He's a real knowledgeable guy and understands what it is that we do with our trucks. He said that Ford doesn't have a specific interval to flush/change the PS fluid. He said the best thing to is to pull a little of your fluid out and look at it compared to new fluid. If there is a distinct color change (ie dark, milky, etc) then change it. A noticeable 'burnt' smell is a good indication that it needs to be changed as well.
He admits, and I agree, that these are pretty basic tests and that an actual change interval should be established. Especially with some of the issues known regarding our PS issues (pumps, racks, etc.) He said that a lot of the service forums that he has looked at have suggested changing the fluid out between 30,000 and 40,000 miles. But that is a pretty long interval especially with what we do with our trucks.
So, my suggestion would be to monitor the color and smell of your fluid. If it starts looking or smelling used, then change it. I personally changed mine at around 40,000 miles last year and again the first of this year at around 70,000 miles (I installed an RPG system at that time). I will start looking at mine more closely after maybe 4 oil changes or after a hard run.