Too quickly.

I had the nexus hub put in about 3 years ago and the cable replaced 2 months ago.

I'm not too sure when the previous cable had started to break down, so maybe it had been breaking down a strand or two for a while. I'll keep an eye on it. Maybe it'll last another year or two but it seems too fast to break.

I think in over a couple of decades of riding, I only had one other shifting cable break before. Of course, that was with a conventional derailuer system,