Now that I have fully rebuild the cylinder heads (all engine gaskets + valve lapping + lash tuning) on my 2007 Tacoma with 124K mi, I am curious how much life I added to the vehicle, approximately. I know that there are other vehicle components, such as the drive train and transmission but cars typically go to the junkyard because the engine fails as it is the most complicated entity and most sensitive to failure.
Realistically, you haven't added any extra life to the vehicle itself. The vehicle is a combination of it's sub-parts (ie: engine, transmission, differential, etc.). Any part at any time can fail for whatever reason. Even the engine is made up of sub-parts to create it's whole. You have definitely given the heads longer life, but you still have the rest of the engine. If an engine will last 250k miles, that's how long it's going to last. The only way to increase the longevity of the engine (beyond proper maintenance) is to rebuild the entire thing. That means bearings, rings, and whatever else may be worn out. Gaskets/seals are a given replacement item when doing this work because in order to get to the larger parts, you will have to put new gaskets/seals (in most cases) in place or you've completely wasted your time.
With proper maintenance, Toyota's are known for lasting a long time. As long as you have good oil pressure, you probably have another 200k you could expect out of your engine. This is, of course, as long as the engine never overheats or some other unforeseen issue doesn't arise. That is my SWAG, though.