That's a question with too many variables-there is no "practical" line in the sand for a vehicle.
A lot depends on your ability to continue to maintain the vehicle so it's both safe and dependable. You also have to be willing to put the extra time, effort and expense into it as age related failures begin to show up. If you're willing and able to do what it takes, you could drive it forever, such as Irv Gordon has done with his 1966 Volvo.
In the past there was a mental barrier at 100,000 miles that many people had-beyond that a vehicle wasn't dependable and should be scrapped. Modern vehicles have made that mindset obsolete, but there are still a few that are stuck on a mileage number as an indicator of a vehicles life expectancy. After all, there are still people that believe that thicker oil is always better and that all oil should be changed at 3000 mile intervals.
You also have to make certain that if you reach your personal line in the sand for a vehicle, that you're not trading one ageing money pit for another. Choose that line wisely otherwise you'll end up with a different vehicle with just as many problems.
Keep in mind that if you're not responsible enough to save up for a newer vehicle you'll end up tethered to payments, which is financially foolish. And whatever you do, don't get fleeced by a lease.