Provided you look after it, I would expect a true condenser mic to last over 50 years
without requiring any major work. Even electret condenser mics (which have been
occasionally known to lose their charge over time) should last at least 20-30 years.
Signs of ageing include increased noise as dirt accumulates on the capsule and
reduced sensitivity as the electret charge becomes weaker.
JRP Music - Audio Mastering and Restoration.