I only remember some rough estimates I read a while ago, but here's what I know and played by on the console version.
Tranq knocks out for about 3-4 minutes, use it whenever you can't CQC the person for whatever reason.
Throws(holding a direction and pressing the CQC button) knock out for only about 45 seconds to a minute, use whenever you need someone out immediately.
Throws while facing a wall knock out for 3-4 minutes, same as tranq
Chokeouts knock out for about 3-4 minutes.
CQC punch combos knock out for a REALLY long time,
I have honestly never seen someone wake up from a punch combo on their own. If you start it from behind, it is only 3 hits, making it really fast. Also, if you do it next to a wall, similer to a wall throw, it will become a 2 hit combo, making it even faster. I would recomment using this as your go to stealth knockout.
While enemies are in any of these states, they can only get up once their timer is up, they get hit by something, or a guard comes and wakes them up.
Hold up-Get downs, on the other hand, last indefinately, but if you get caught and go into alert, they will get up on their own, and can also be risen by other guards as well.
I'm not actually sure on the stun gun timer, don't use it that often.
I actually got a lot of this information from this video http://youtu.be/xMptPmlL6bQ kind of long and awkward but has a lot of information.