IMO, you can use less time. You will reach max temp in about 15min or less. I have read somewhere that there is a concern that needlessly running Prime95 et al. on a highly OCed CPU for extended periods of time will needlessly increase electromigration. I don't know if this is true but it certainly makes sense. If you have a stock clocked CPU I don't think it makes much difference. If you can run prime small FFTs for an hour and Prime95 blend for an hour, then your machine is stable enough. This is my opinion only, I'm sure many do not agree.
I agree. I've always thought the amount of time to stress test a chip is the same as the maximum amount of time you'll use the computer for under stressful conditions. So for me, I usually do a 3-4 hour stress test, since i never use my computer for longer than that at any one time.