Once again, you have failed to do even a minuscule amount of research, or you'd see that diazepam has a notoriously short duration, at the clinically relevant doses.
A little more research and you'd find that this is because it's extremely high lipid solubility leads to rapid redistribution from adipose tissue.
And if you knew the slightest thing about pharmacology, you'd know that half-life is often irrelevant to the duration of a drugs effects. This is true for many drugs, including all benzodiazepines.
Seriously, anyone who has ever taken both diazepam and clonazepam will tell you that the idea that diazepam lasts longer is a fucking joke.
Please, please, PLEASE people, stop using the term "half-life" if you don't understand it.
Not to be rude man, but seriously? This is 2 posts in one day where I have to clear up this bullshit about "duration vs half-life".
I swear, I am going to put a link in my signature, to a benzodiazepine chart, showing that the duration of the respective benzo's (and therefore the respected dosing) is irrelevant to the half-life.
In the mean time, take 10 minutes to read through my previous posts, it shall enlighten you