I know that the Dmax doesn't have hydraulic lifters, and has adjustable tappets. I can't seem to find out how frequently they should be adjusted. Does anyone have any idea? I even have the factory service manual, which shows how to do it and gives the clearance, but doesn't tell how often.

no service required

If it doesn't have hydraulic lifters, how does one adjust for wear?

They are adjusted from the factory for the "life" of the engine. Take that for what it's worth.

Roller cam and lifters, wear is minimal. Also, the base circle of the Duramax cam is HUGE, so that also helps.