The lifetime of an autotransformer is indefinite, and does not necessarily end with a short circuit. Barring a transient on the mains, water leak or some other external event, there is no reason it should fail. However, if the low-voltage part of the winding were to open (as well as if the high side were to short), you'd also be faced with full line voltage across it! If safety is paramount, use a transformer with isolated secondary -- the failure mode is almost always zero-voltage output, rather than full line voltage across it (though it is possible that extreme overheating or over-voltage could burn through insulation).
Another advantage of an ordinary transformer is allowing a ground to be placed anywhere, avoiding likelihood of a ground-loop issue and reducing chance of electric shock.