Take a junk bike frame and try to saw it in two with a brake cable. See how long it takes you. Now measure your brake cable pull. Figure out how many cycles it would take to match your first experiment.

To be honest with you, I've never had a bike frame that didn't last longer than I wanted it to last. I'm always lusting for something new long before the old one wears out. That's why it's so easy to find a junk bike frame for the first experiment.