I think the diaphragm probably deteriates with age, not mileage. The problem I experienced was mechanical wear, and gave me adequate warning. When it gets to the danger point that Benzmac warned about. You will be hearing a snapping noise from the pump.
Benzmac may tell us that he has seen them go without warning, but I guess I heard the warning, headed it and was lucky.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in