Depends on which version. The latest version, you want to usually select Blend Test. It starts out with slight cpu useage, but will eventually go up to 100% load. It stresses ram and cpu alot...it's a good combination. If that fails, maybe try the lower FFT test's. Just let it run as long as it can. When it's running successfully, the icon in the bottom right will remain red. If it fails or halts, it'll turn yellow. Anywhere from 8 hours to 24+ hours is good in my book.