For SN, which has a shorter time-frame to CTB, any benzo would work (since, the length of action is not important).
When to take it is dependent upon the particular benzo and its onset of action.
- Rapid onset (<15 minutes) are diazepam and flurazepam,
- Intermediate onset (15-30 minutes) are librium, xanax, clonzazepam, and halcion, and
- Slow onset (30-60 minutes) are temazepam and oxazepam
However, you should also consider the Peak onset for the particular benzo you are going to use. This is when you would be more likely to be "knocked out" and for most benzos this would be about an hour (guessing you will take it orally and not i.v.). But some take longer for Peak onset (such as librium at 2-4 hrs). Also, it can depend upon your history with the bezno. So you will have to do some reasearch for your particular benzo.