I'm describing what PC makers buy and sell for retailers, which equates to replacement cycle given the same price, not CPU form-factor and evolution.
I currently own a Dell i660s-775BK Celeron G470 that I paid only $199.99 sale on November 2013. Then this model was discontinued on February 2014, and Dell replaced it to i3646-1000BK with J1800 as the new model for 2014. Due to low-cost of production, this same model successfully ran over a year with no change.
I did buy and try the new i3646-1000BK before, but returned it because it's only half the speed from G470. It was also cheaply-made, no SATA III (my G470 has one using B75 chipset), and no internal power supply (only external).
Because of that, and my earlier saying why new computers are NOT necessary faster anymore, I'll be hanging on to my G470 for a while.
About me as a consumer, I'm a HUGE fan on buying only the cheapest PCs there are available. I'm not interested in looking any of the Core i3s and above.