I have seen 6-24 hours. Best thing to do imo is let it run, go to bed and work. That is a solid 16+ hours. I have been pushing my 2500k this weekend and I had crashes on the 1.5 hour mark and 2.5 hour mark. (give or take 5 mins)
more time is more stable for system
In actual use, you will never push the pc that hard for that long.