To echo Larry's experience our '74 240D never had a single valve adjustment in nearly 200K miles of driving as best as I could tell. It was more out of ignorance than neglect. When we bought the car (new in '74) the dealer said, change the oil frequently (every 3K miles) and fuel filters much less frequently and that's all it will ever need and that is all we ever did. In fact, I don't even recall ever changing the fuel filter. Of course now I know better but it goes to show that they can easily go well over 15K miles between adjustments
Realistically, if you did it every 30K it would probably be just fine with one caveat - if you do an adjustment and find the valves to be extremely tight then re-doing it after 500-1000 miles isn't a bad idea - but after that I would still go 30K before checking them again.