The point of Prime stress testing is to run through all the tests. Time is not the sole reason. It takes ~10 hours to run through all the tests in prime.
You can't just say, I only game for 3 hours at a time, so 3 hour stress testing is all I do. That is fail logic to the max. For the 3 hours you game, you don't know what parts of your system are getting stressed. Some instruction sets don't use ram and only use l3 cache, others use ram, others go back and forth between ram and l2, etc.
It's like a car. If prime was stress testing a car, then it would stress the engine and transmission 2 hours on first gear, 2 hours on 2nd, etc. If you only do 4 hours, you never stress 4th or 5th gear. The problem with IBT is that it just stresses the same instruction set over and over. It's good to get temps, but doesn't really tell you anything about the different ways your system can be tested.