How long to verify stable?

A couple days ago I ran the OC on my 8400 up to 4Ghz and primed for about an hour on small FTT's and it was fine so I switched and fired up folding. After a couple of odd issues with folding and a DOD:S game crash I retried prime and it failed in about 1 1/2 hrs.

My question is how long do I need to run prime to truly verify that it's stable for 24/7 folding and which type of test should I run?

I usually run prime for about 12 hours, but F@h is the best thing ive found for stress testing. Even after ~30 hours of P95, F@h still crashed, but after bumping the voltage up one more notch ive been fine ever since.

Well with 24/7 folding in mind you probably don't want to run a stress test long enough to find out if it's 100% stable. I'd say run the small FFTs for about 24 hours then start folding. I had my Q6600 folding at 3.85Ghz for a few months and one day I randomly got a EUE and a BSOD. Backed down to 3.7 and it's been fine ever since.

I have found that at least 18 hours is the minimum for stability testing.
When using Prime, I use 'In place FFTs' as I found that it produces more heat and stress than 'Small FFTs'
Temps have always been higher runnung In Place FFTs
eg, I was testing at FSB 385 with Small FFTs for 8 1/2 hours, I stopped that test and started In Place FFTs and it had an error in less than 30 mins.
When monitoring the temps, In Place FFTs has always show higher temps.

Last night I had Prime running with FSB 385 and VCore 1.44375, load was 1.448.
Room temp varied between 19 and 25 C.
Prime failed after 14H 45 Mins

I currently run it at a stable 370FSB with Vcore of 1.39375 and a load of 1.336
With a room temp of 22-23C
27 27 25 27 Idle temps after a few hours of being on
49 49 45 45 Prime 4 threads after 4 mins
48 48 40 44 Prime 3 threads after 3 mins
43 43 38 40 Prime 2 threads after 3 mins
38 38 36 40 F@H - 2 instances, 1 for the CPU and 1 feeds the GPU (55% CPU)
See sig for system info

I agree, you should run AT LEAST 12 hours. I ran "folding" as well for almost 6 months, zero problems.

everyone is different in that respect, some say 8hrs, some say 12hrs, some say 24hrs. i have a preference for doing it 24hrs since i tend to run my system 24/7.

I prefer to see if its stable beyond 24 hours. I will trust 12 only so. far for rock hard stability check 24 hours.