Does a 35 year warranted asphalt shingle have a lifespan of 35 years or is a 20 year lifespan more appropriate? I have been told that the warranty does not apply to the expected life of a shingle.

Yes and no. Given the proper conditions shingles will last an indefinite time. I have seen homes with 30 to 40 years of shingle use and still not be in bad shape. I have also seen shingles on homes that have lasted only 10 to 15 years. Probably the worst culprit for premature lifespan are homes with improper ventilation, excessive moisture and
lack of insulation. Make certain these three areas are satisfied and you should be able to gain long life for your shingles.
GOOD LUCK!!!