How long will my video card last...

You could put a large slow fan inside the box directing air towards the GFx card. I did same with my fanless card and it brought the temperature down some 20 degrees and was pretty silent. I also took a noisy fan off an old Radeon 9600 Gfx card and worked fine, then I upgraded, I still have it to use for diagnostics on AGP machines and it doesn't get hot.