5 years is usually the minimum I'll keep a car. That is my day to day use one. I have an avalanche and Thunderbird sitting in the garage. I also have an impala thats been out of commission for a while now. But that's being shipped off the an old friend who's willing to give a good home.
For me I just usually wait until I fall out of love with it. When I dont feel like driving it anymore or I just get bored of it. I only buy cars that I enjoy owning but when I dont enjoy it anymore thats a pretty clear sign for me to move on and get something new.