Folks in Hollywood change everything frequently, including spouses!
As per the previous posts, if the ongoing repairs, for 3 years say, are suchg that they exceed the average annual 5 year (=50% of purchase price) depreciation of a new car, you should trade. So, on a $20,000 car, the average 5 year depreciation might be 50%x20,000/5=$2000. As pointed out,that can be a long time, considering the price of new cars. On a $40,000 car, that would be about $4000 per year in repairs and maintenance.
We have may posts of drivers who have owned the same car for 20 years, driven 500,000 miles or more and still have an economical, reliable vehicle.
Having said that, there are other overriding considerations:
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The car becomes unsafe, such as through underbody rust
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The car can no longer meet emission tests.
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The car was not too reliable to start with and the frequent repairs(although not excessively expensive) make it unsuitable as a daily driver.
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The car becomes unsightly, and affects the way your employer judges you.
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You work in sales and image is important.
If you worry about the opinion of your friends, time to get other friends. I drove an immaculate Chevrolet Caprice for 19 years and was constantly asked questions about how to make your car last.