How long does thermal interface compound remain effective?

As long as you do not take the joint apart, it will be good for many years. Of course, this presumes you assembled the joint correctly in the first place. The object is to "fill in" the crevices in the surface so there are no gaps for air, right? Surfaces that appear smooth to your fingers are really rough when looked at under a microscope, looking more like a mountain range than a mirror. As a result, two surfaces that appear to be in contact really are NOT, with only the high points touching. The paste fills in the gaps so there is no air space and THAT is why it works as it does, to conduct heat from one surface to the other. The paste is oil based so it does not "dry" as such, but over time, it WILL "thicken" as even oil vaporizes. The only area avail for oil to vaporize is the edge where the surfaces meet. The thicker the layer of paste, the more area is exposed at the edge for oil to vaporize. Still, the rate of oil vaporization means it will take many YEARS before anything significant will happen to "harden" the paste. You have nothing to worry about.