My current car is a 2000 Ford Focus. I bought it brand new. I love this car and will drive it until it won't make it out of the drive way.
This is my second new car, my first was a 1989 Ford Escort and I wish I would have kept it a few more years.
I love the Focus, but I could have gotten a good 3 - 5 more years out of the Escort. I had the Escort for 10 years. That is the minimum I would keep a car, I am planning on keeping the Focus longer than that.
However, if you get to a point where the repairs are more then a monthly car note, then I think it is time to go for a new car.
My next door neighbor leases his cars and has a new one in his driveway every three years. However, that means he always has a car payment.