I was just wondering, when you are putting your computer to the test how much voltage variation is really too much? Where do you draw the line and say my PSU is taking too much punishment? Of course there will be some core voltage variation, but whats an acceptible amount?

Normally as long as the variations are less than 10% then you are fine. The max that the 5volt can fluctuate for our example, is like to 4.85, to 5.5. A little lower or higher than that is fine depending on what it starts at. A good rule of thumb is to not let the voltage rails fluctuate more than 10% from where they started at. Speed fan and motherboard monitor 5 will both tell you your temps and voltages.

10% as Drago said is correct, but I would consider that the outer boundry
5% is what I would aim for regular PC use
for good overclocking you want 2% variance

IMHO