I tend to hang on to a car until it starts having problems or is no longer reliable enough to trust on long trips. If I had a car such as the E90 M and I would really like it (I would drive it nicely to keep the mpg's higher except for when the time to "go fast" comes up) I would still keep it a long time. Having the last year of production helps the reliability somewhat too, but nowadays with modular assembly and multiple suppliers its more a question of quality control and design. Most of the reliability issues of the past are not there since all manufacturing has improved 5% - 10% a year if you read the data by JD Powers.
I've noticed BMW people do change cars quite a bit, so its fun no matter what! But NRG be careful - if you sell your beloved E90 M3 you may wish you didn't in the future because there may be nothing like it ever again.