That is not an answer that is easily quantified. There are many variables which will affect how long a particular engine will perform. I would only offer that the new generation 6 chassis should far outlast any of the predecessors based solely on the fact that the entire gear train is metal, no plastic.
Im pretty sure he is referring to the "true" annie..the Generation 6 Big Hauler..since that is the one most likely to be for sale in a set at this moment..the newest Gen 7 Big Haulers aren't available yet, and probably wont be before Christmas..
But either way, any "current" Big Hauler is a quite robust machine..no one can really say for sure how long they will last though..because owner care and maintenance is a major factor..but overall, they are an excellent locomotive..
treat an annie well, and she will run for a long time..
Scot