No one can tell you the answer because a lot the effect is based on your tolerance and the individual way you react to the drug. With that being said, my guess is around 10 hours. Valium is a longer acting benzo than xanax so you would get more than 8 hours of sleep most likely. Once I overdosed on the clonazolam and took the equivalent of probably hundreds of mg of valium, I woke about 36 hours later. The doctors thought I had brain damage and a major reason was that it took me very long to wake up.
I strongly advise you not to purposely OD. It can be a very unpleasant experience. My jaw got dislocated sometime after I ODed and it was left open for 40 hours it would have been closer to 48 if I had not put it back into place as I woke up. In addition I was described as passed out and doing nothing for hours before I went to the hospital and after the person who took me the hospital. The next time they say me in the ER I was thrashing around screaming. All in all an unpleasant experience. If this goes wrong just know you most likely won't be getting adequate care.
Basically you run the risk of an extremely unpleasant experience and it being on your medical records for the rest of your life, influencing future interactions with doctor and care.
