Benzodiazepines alone do not generally yield very high mortality rates. They are useful as sedatives and as components of other methods, but I don't really think I would use them alone as a method.
Questions like this can often be researched by looking at studies on PubMed/National Institute of Health (NIH). Obviously these sites will likely not directly state the amount considered to be a "fatal dose", but you may find studies regarding individual cases, prognoses, and outcomes from different amounts of different drugs. Earlier I was looking at a case study on PubMed of a girl who attempted suicide by taking 5000mg of modafinil, it was interesting.