every mfg has its own recommended oil change interval.
most of the trucks i've done in the past used to be an average of around 200 to 250 hrs. Since the technology has improved drastically over the past few years some mfg's have increased thier recommended oil change intervals in some cases. I've seen some go as far as 500 hrs.

You just need to check with your dealership and see what the recommended interval is by mfg's recommended specs based on the truck you are servicing.

Of course as was mentioned, the envrionment plays a part in this as well, normal conditions you would just follow the recommended intervals. Harsh or heavy use environments of course you would monitor the oil condition and change the interval based on that.