If the system is going to fall over easily, it will usually do it within a couple of hours.
To be totally honest, I gave up with all of the "stress" testing. All I do now is run Memtest for a couple of passes to make sure the RAM is OK and then, if I am overclocking a system, I suck it and see.
In everyday computing you are very unlikely to put your rig through as much effort as the stress tests put it through so why try and make it fall over and possible kill components in the process? Why spend days testing when a sudden electrical spike or dodgy download can make your system fall over anyway?
If you are overclocking, keep on eye on your temperatures and get on with using the computer. If it does fall over tweak a few settings, re-boot and try again, just like learning to ride a bike. Surely, that's part of the fun!