I have a the PS box too. . works great. It will set a code on your truck if you let it idle too long. . something about vech speed does not match RPM. The code does not make the check engine light come on and has no ill effects.
As for how long to idle your truck... I have idled mine for a couple of hours once when I took a little snooze while it was warming up. At work (in the Alaska oil fields) it used to be common practice to let the trucks run over night or longer to keep them running in real cold (-40 deg) weather. I know for a fact that I had a ford diesel at work that was not shut off for well over a month. I am not saying that was good for it, but it kept running for a couple more years till the company got rid of it. I would think letting it run for a couple of hours on high ilde would not be bad, I don't think I would make a regular practice of it, but once in a while would be okay.