Time vs Mileage for Part Replacement for '91 Coupe Quattro

["Christopher G. Gharibo, M.D." <gharic01@mchip00.med.nyu.edu>]

I was wondering if some tune-up parts truly need to be changed on a time
interval, rather than pure mileage.  		

I would assume engine oil (7500 miles/6 months) certainly needs to be
changed based on mileage/time interval, however, does the same apply to the
air filter, fuel filter or spark plugs which are recommended to be changed
every 2 years or 30,000 miles.

Doesnt mileage factor into the replacement of an air filter, fuel filter or
a spark plug to a much greater extent than time?				

If an air filter, fuel filter or a set of spark plugs have been in a '91
Coupe Quattro for two years that has only accummulated about 10k miles
during that time, do they really need to be changed or can any one of these
be left in the car until the 30,000 mile mark?

Thanks.

Chris