My rules for when to buy a new car:

It's time to buy a new car when.....

1) the repairs will cost a small fortune, more than the car's trade in value is.

2) I can no longer rely on the car running, or do not trust it to take me on long trips.

3) it smokes, stinks, rattles, is unsafe, or in general looks/acts like a running junk yard escapee.

4) it's needing alot of minor repairs but the total of them will cost a small fortune or repairs will cost more than the payments on a new one.

5) When any of the above are starting to be realized and there's a super, super sale at the car lot.

6) I can still get some money back from my car without having to PAY someone to tow it away as junk.

I average about 10 years and over 100,000 miles on my vehicles when I trade them in.