68.2 % of the data is within one σ, either side of the mean
95.4 % of the data is within two σs, either side of the mean
99.7 % of the data is within three σs, either side of the mean
Therefore, 0.3 % of values lie outside the range of value of three σs.
We can say this is representative of the chance that a value (in this case, the lifetime of the fan) will exceed the mean value by more than three σs.