5-7 years for a model seems about right to me, at least through 2020 or whenever the 35mpg gov't mandate comes into effect. BMW is an excellent example of this ethos. The E36 3 series was on the market from '92-'99, 8 years (including '92). By the time BMW retired that model, Audi's B5 platform was on the market and even though it wasn't as focused a chassis as the Bimmer, it looked bang up to date for the late '90's. So BMW releases the E46 chassis, which stays on the market, in one iteration or another through '06, or 7 years. Again, Audi comes with the B6/B7 chassis, and by '06 with the possible exception of the M3, the E46 chassis (and I'm including the exterior in the 'chassis' definition, even though I know it's not technically accurate) begins to look a little dated. So then we get the Bangle-ized E9X 3 series and even though it's a love it/hate it look, you put an E46 next to it and it just looks kind of old. Not bad, by any means, just old. You can tell it isn't a new model. It's all about competition. Audi and BMW stagger their chassis updates so they can see what the other competitor is going to bring to market. All direct competition car companies do that, and they've seemed to settle on a 5-7 year model run as the number that makes sense. If the new Z runs until 2014/15, then it'll have time for one more model roll out before it has to become part of Nissan's fuel milage strategy. Long post I know, but it's an interesting subject!
