Well the half life is very long (about 20-80 hr's depending on the person) therefore it will be in your system much longer than most other benzo's....but, coming from a personal user of klonopin(prescribed, not off the street) the effects are only noteworthy for about 6-8 hours like captainheroin said.....allthough it is still working long after that, its just in such a subtle way you cant tell.
*Sometimes when I save up a few days worth of my klonopin doses and then take them all at once (around 3-6 mg's), I get very "loose" and relaxed and most the time very tired at certain points causing me to take a couple naps.....and I will eventually fall asleep(quite easily)....but when I wake up I can still feel the effects....relaxed, kinda like I just got a full body massage.*(meaning... depending on your dosage you can still feel it after a good 20 hours or so)
Allthough, one problem is that when that happens, I usually get a bad case of the hiccups...I dont know why.