It really depends. there are a lot of things that affct a magnets retention of magnatism. Obviously the magnetic material is one, then theres temperature, temperture variation, imapact and shock (if the pickups been taken out or dismantled), presence of other magnetic feilds (they're all over the place and over long periods of time, they dont need to be as strong)
All told the degredation of a magnet over time is quite unpredictable unless you have data on how that magnet degrades over long periods (years/decades) with all possible variables controlled (never been done) and even then it would be vary hard to determine, simply because you dont know where its been or how its been handled in its few decades of life.
Speaking as a physicist here, there is no way you can quantify the degredation of a magnet over long time scales if its never been baselined in a controlled test, and theres no way you can ever give a set answer to this question for any given magnet and say "thats it, all the time" because you dont know how the other factors have affected it.
All you can do is measure the magnet now. If youre lucky maybe you can compare it to one from then (near meaningless result but it would probably convice some) or a large sample from then, but that would be a statistical assesment because you arent measuring the same magnet, so it still doesnt tell you anything if you dont know what that magnets been up to in that time.
Edit: and the data for 100 year lifetimes is also very iffy since it'll be based on extrapolations from very much shorter periods of time. The errors in such large extrapolations are significant, and there is the possibility that the real behaviour undergoes unpredicted changes in trend.
