Anyway, we were about five miles from the car when I heard the usually taciturn Larry say, "Damn!" which from him was a veritable torrent of verbiage. Turning around, I found him staring at one of his boots. The sole had delaminated, leaving him with a gape-mouth boot. Some adhesive tape sort of fixed the problem until we could reach the car.
That's one potential clue that it's time for new boots - the sole delaminates. Sometimes repairable, but often a sure sign you need a new pair of boots. Other indicators: The leather is so soft that they no longer offer good support, the stitching has completely blown (although that can be repaired), and the boots have shrunk a half-size, which can indeed happen with really old boots. If you pronate, as I do, you might find the boot has canted 15 degrees to one side - another sure sign to move on. Finally, I had some old Galibier mountaineering boots with a steel shank that rusted right out of the boot! That was repairable too, though.
So, the answer is: Whenever YOU think it's time for a new pair. I think you'll know it if they get so soft they no longer perform they way you want them too. And really, a good-quality pair of leather boots, properly cared for, is as close to a lifetime investment as you can make these days.