I agree with you about the virtues of Salomon shoes, which are certainly great. I had a pair of their hiking boots for a long time before switching to an LL Bean model, and they treated me well.
Too well, I think, because I, too, wore them too long. Surely there's some rule of thumb for trail running similar to the 500-miles-for-road-shoes maxim.
I suppose it depends on the shoe and where you go, but I wouldn't think you, Andrew, would want to do heavy training in any given pair of shoes for more than two or three months.