There are many answers to this question. I have usually seen that 2 hours seems to be the bare minimum with 24 being a max recommended. Personally I am a bit more moderate than that and I would say if you are testing for long term stability I would go for 12 hours or maybe start it at night before you go to bed, check on it in the morning and if it is still going strong let it continue through your work day.
they aren't overclocking. if you did get a bad cpu you would figure it out pretty quickly. no need to run prime for 24 hours. run it for 15-30 mins. intelburntest uses linpack which is what intel uses to test cpus.
On a new build, If Im not testing and OC, I personally start Prime, and go to bed. If it's still testing when I get up I consider it to be a stable build. I go a bit extreme with my OC testing, I do a full 24Hr run, if it craps out I don't consider it to be a stable OC.