Everyone has their own priorities and preferences but I think is the most crucial factor would be dependent on the financial capabilities of the person. If selling the old car, that will mean committing to a buying a new car. Hence, the question of "When is the best time to sell the OLD car" can be rephrased as when is the best time to buy a NEW car, which I suppose is the same thing unless the car that's going to be sold is an excess car which can be sold without the need of replacing it with a new one.

For me, it used to be 5 years after the loan was completed, particularly since the car was able to fetch a rather high resale value even when traded in to the 2nd hand car dealer (New around RM115k, after 5 years sold at RM62k to 2nd hand dealer). For my current car, I am hanging on to it longer since it has poor resale value and for the fact that I enjoy the drive and performance of the vehicle. However, after 8 years the car started to give problems with high maintenance and frequent visits to workshops so it's going to be soon.

In summary this is what I think regarding the best time to sell your car:-
1. When it can still fetch high resale value and you have the interest of getting a new car
2. When the car starts giving you problems with high maintenance repair bills and frequent visits to workshop

Well, as most have known cars are a depreciating asset, people don't actually plan on it too much in terms of making a "financial sense". If want to minimise losses, there are other ways such as buying cheaper cars, buy cash or take the short 2 year loan etc. rather than buying expensive cars, paying a small downpayment and stretching the loan to 9 years etc. One can focus on other areas such as investing in property or FD etc. to counter balance the "losses" in the sale of your vehicle.