In terms of duration of action, Ativan (lorazepam) is considered a medium-duration benzodiazepine. Valium (diazepam) is considered short-duration; Klonopin (clonazepam) is considered long-duration.
Simply put, short benzos come on quickly (i.e. reach peak plasma levels quickly, usually 20 minutes or so) and their effects end quickly (4 to 6 hours; for some even less). Medium benzos can take an hour or two to reach full effect, which tends to last 8-12 hours. In my own experience, Ativan takes 1 to 2 hours to reach full effect (less if dissolved under the tongue

) and lasts about 8 hours. A very useful drug.

Long-acting benzos like Klonopin or Librium (chlordiazepoxide) can take 2 or more hours to reach full effect. Klonopin can stay in effect anywhere from 16 hours to 52 (!) hours (you could have a beer the next day and end up OD'd!); Librium acts for days on end.
Short benzos are mainly used as sleep aids or muscle relaxants nowadays; medium benzos tend to be used for anxiety; long benzos tend to be used as anticonvulsants/antiepileptics.
Note: This is a basic short summary, and I am NOT a doctor of pharmacist.

The Web has all of this info and more.
I found Ativan very, very useful (too bad my current doc is too leery of them). Just take them as directed and don't mix them with alcohol. Also note that the "smooth" action of benzos means that after a few hours you may no longer realize that you're being affected by a tranquilizer-- though you are. Care and common sense will keep one safe though.
