If you're going to have stability issues, voltage spikes, heat issues or a bad chip, it won't take more than 8 hours to find out. As for getting the thermal paste worked in, it really does not take that long at all. Out of the hundreds and hundreds of PC's I've built, I think the longest I've tested even OC's were 12 hours. I hate to give credit to someone here, but it's like @Ghost who says it's NOT correct to leak test a PC for 24 hours as you can burn things up that way. Stressing your CPU for long periods of times in ways that synthetics do is a BAD idea. Not even folding stresses it to those levels.