In 32 years/300K+ miles of street riding, I've usually (but not always) been able to replace sprockets with every-other chain replacement. I judge it by actual observed wear, not the rule of thumb. But I'm not particularly hard on my drive trains, keep my chains well-lubed and adjusted, buy top-quality chains, and don't ride off-road (though I do ride in rain, and on salty roads in winter.)