It depends on the vehicle. I typically start to get antsy/nervous around 180K. For me, the time and miles at that point mean the dashboard is probably starting to crack, the seats may be showing some wear, and if something as minor as the A/C breaks, it could be more expensive to fix than the cost of the vehicle.
with my current rides, I think the car in my signature could very well last forever as it has been totally bulletproof and there are tons of parts available for it. Some of my other cars have limited OE parts still available, making long term maintenance a chore. I don't want to buy junk parts and/or salvage parts if at all possible. I had to buy a junkyard rear diff just to get a flange to replace one that was damaged.
180K is usually the time I start to try and decide if I want to keep it perfect, or if I should save the 'keep it perfect' money to put towards a replacement. Do I really want to do $1,000 in tires and $1,000 in worn suspension parts and $500 in brakes and another $500 because I should replace the radiator and water pump, or should I leave that 3K in the bank and drive it with cheaper tires, suspension that is starting to make noise and possibly get stranded at some point in the next year if I don't replace the radiator - these are known to crack and/or mix ATF with the coolant - which causes transmission destruction. And if I do put the 3K in the car to keep it perfect, if the transmission fall out this winter, that's thousands of dollars to replace the transmission in a car that probably has 50K left in the engine.
So - 180K is my nervous point, and I'm almost there on two cars currently.