Well for me, I dont really have a mileage set, but I will go off of years that I had the vehicle also. The last truck I had I made the mistake of lifting it and decking it out with all kind of accessories, then woke up one morning about 4 months later and sold the truck and got the truck I have now.
So now if I dont do anything within the first year or 2, its gone to sit like it is. Luckily I bought the f150 used last year, so I still have time for the lift and stuff
But I know that probably wasn't what you were getting at, so honestly I probably would not put much money into a vehicle after around 80,000-90,000 unless I planned on keeping it. But that number would probably go up if I averaged more miles per year or if the 09+ f150's dont get to looking any better. But with all of that being said I have a 92 f150 with 237,000 on the clock that I put a lot of mods into.