All The Time
Rendering, video encoding, CFD, simulations,...
At the same time with no slowdown of the interface so I can do other stuff while waiting for them to complete. I can easily pin all 8 threads at 99% with this stuff and do so regularly.
When I can render graphics like Avatar in real-time, I might be happy. But then I could start building entire planets at micro-metre resolution,... There's no limit to the amount of processing power one could use if it was available.
And, yes, I know that is not typical, but we are talking abut ALL desktops.