The answer is that there is more than one half life. Because Benzo's are fast acting, you look at the Distribution half life. That's around a few hours max for Diazepam, it's typically one hour though. It also has an Absorption half life which differs slightly.
The half life you have been thinking of is the Elimination half life which is just a measure of your body getting rid of half the dose.
Meds like antidepressants are slow acting meds and so other than their absorption rate and peak time, we only think of how long they are in our bodies doing their work. fast acting meds can be largely doing nothing for the majority of their Elimination half life.
Pharmacokinetics
Absorption
After oral administration >90% of Diazepam is absorbed and the average time to achieve peak plasma concentrations is 1 – 1.5 hours with a range of 0.25 to 2.5 hours. Absorption is delayed and decreased when administered with a moderate fat meal. In the presence of food mean lag times are approximately 45 minutes as compared with 15 minutes when fasting. There is also an increase in the average time to achieve peak concentrations to about 2.5 hours in the presence of food as compared with 1.25 hours when fasting. This results in an average decrease in Cmax of 20% in addition to a 27% decrease in AUC (range 15% to 50%) when administered with food.
Distribution
Diazepam and its metabolites are highly bound to plasma proteins (Diazepam 98%). Diazepam and its metabolites cross the blood-brain and placental barriers and are also found in breast milk in concentrations approximately one tenth of those in maternal plasma (days 3 to 9 post-partum). In young healthy males, the volume of distribution at steady-state is 0.8 to 1.0 L/kg. The decline in the plasma concentration-time profile after oral administration is biphasic. The initial distribution phase has a half-life of approximately 1 hour, although it may range up to >3 hours.
Metabolism
Diazepam is N-demethylated by CYP3A4 and 2C19 to the active metabolite N-desmethylDiazepam, and is hydroxylated by CYP3A4 to the active metabolite temazepam. N-desmethylDiazepam and temazepam are both further metabolized to oxazepam. Temazepam and oxazepam are largely eliminated by glucuronidation.
Elimination
The initial distribution phase is followed by a prolonged terminal elimination phase (half-life up to 48 hours). The terminal elimination half-life of the active metabolite N-desmethylDiazepam is up to 100 hours. Diazepam and its metabolites are excreted mainly in the urine, predominantly as their glucuronide conjugates. The clearance of Diazepam is 20 to 30 mL/min in young adults. Diazepam accumulates upon multiple dosing and there is some evidence that the terminal elimination half-life is slightly prolonged