Toyota Camry 1991: When should I sell?

NOT an easy one, because it isn't possible to predict when it will become uneconomical to maintain. The reality is that the higher the mileage, the more likely it is to break down, but it may not. The Camry generally doesn't give too much trouble, so I wouldn't rush in and sell it simply because the odometer is climbing. If you want to update, then it's probably better to do it sooner rather than later because its value is decreasing. I get the feeling you're happy with the Camry, so keep it.