How long is an "updated" roof on a house supposed to last in normal coniditions?

When I use the term "updated" I mean nailing new shingles on top of old shingles. Thee is a house I am interested in purchasing. The real estate listing advertises a "new roof." However, I know that the roofing company did not replace the shingles and the wood under the shingles. They just nailed new shingles on top of the old ones. Around here they call that a "new roof." LOL I call it an updated roof. Anyway, how long is an "updated" roof supposed to last under normal conditions, meaning no major hail storms, just the occasional thunder storm and average or above average snow fall?