I ran mine at 400/1100 for almost a year with no issues. I think the guy I sold it to was planning on running it at those speeds also. The GT is pretty much an underclocked Ultra, and most will run at stock Ultra speeds just fine for many years. That being said, some won't... YMMV...
Sorry, but there just isn't some formula where you take the:
Life of Card - [(C * MHz) / FPS] = New Life Expectancy
You're trying to get a definite answer to something that cannot be guaranteed.
I also don't want to have to keep adjusting the clocks like setting it high for gaming and down to normal for everything else.
...you don't have to do that anyway. Notice the 3D and 2D clockspeeds. You only use 3D clockspeeds when you are doing something 3D.