I have a very nice BMW hardtop convertible that I just purchased, but I refuse to trade in my 2001 Jeep Grand Cherokee (60th anniversary edition)....I've restored every bit of it and it has 88,000 miles on it...not even broken in yet!!!! I use it only in the winter....If you don't maintain your vehicle, trade it in and have a nice presence!!  if it works well and looks nicely, and you like it, keep it.... 

When I get a wild hair as my bride would say :)

Mileage is my big consideration. I'm about due for a new Subaru. 

We don't drive clients often so it's more personal than business needs that weigh in our decision. If the car is in good condition I keep it unless there's a reason I need a different one.

More like when it quits running...A car does not have to be fancy for my back roads it just has to run.

I'll say my last 2 car purchases were total maintenance issues.  The first one the muffler dropped and lead to my first brand new car.  The most recent, after I got my car back from that GM ignition recall it was going to need at leat $1k to pass inspection.  $1k is a down payment! =)

Since being in real estate, I've leased my cars - every 3 years. Keeps me in a recent model car with little to no maintenance.

Definitely when it becomes a maintenance nightmare.

It's always been an emotional decision for me, as the vehicles tend to last longer than my desire for them.

It can be on a whim and happen instantaneously Paul S. Henderson. Mileage usually is not a concern because I know where they've been.  

I'm not at all a car guy, for me it's a means of transportation - when maintenance becomes and inconvenience, it's time for a new one!

Maintenance requirements!

When I find myself walking up the drive, seeing the vehicle and NOT saying, "Wow, that's an awesome ride."

Condition and function