Residential internet rate competition and pricing strategy

In the SF Bay Area, Comcast and ATT always have good introductory rates ($40/month) for internet that expire (to $75/month) and hence incentivize a switch to the other one. On the phone the rep always acts shocked that a customer would switch based on price. Presumably this pricing scheme only works because customers do not switch right away after the rate hike. How long do you wait after your discount expires to switch? Does anyone on HN have insight into these pricing schemes?