IO - Instant-On is a form of police radar that is specifically used to target a particular vehicle or series of vehicles. The nature of IO is not so much the transmission time of the radar, it is the time when the transmission starts (at a specific target, often at close range).
Instant-on is always manually triggered by the officer and lasts anywhere from 0.500 seconds (half a second) to longer.
I believe more typical instant-on transmissions last from about 1.0 second to nearly three seconds in duration.
Quick trigger is merely a shortened or abbreviated form of IO, lasting only around 0.500 seconds to no more than 1 second. Quick trigger can be/is used to beat out slower radar detectors that require a radar transmission to last for one second or more before alerting.
It is especially important to realize that downstream reflected radar often appears for brief periods of time along the same order of QT radar which is why quicker radar detectors will tend to outperform their slower counterparts, even those possessing increased levels of sensitivity, in these circumstances.
The best radar detectors for sniffing out these brief radar signals are the V1, Whistler models (with filter mode), Beltronics
STi-R /w RADAR OFF (and/or Ka band segmentation).
Beyond three seconds of radar transmission tends towards CO constant-on mode.
POP-mode is merely a processor-controlled extremely brief transmission of radar ranging from 16ms (0.016 seconds) to 67ms (0.067 seconds). It is not an IACP approved method for obtaining speed and tickets can't legally be issued with the mode. Typically if POP is used to pre-qualify a speed, a more normal radar transmission mode will follow that provides the necessary tracking history. These more typical IO transmissions would then be detectable from radar detectors that can see them.
POP: 16-67ms (0.016-0.067 seconds), targeting particular vehicle(s)
Quick triggered instant-on: <1000ms (<1 second), targeting particular vehicle(s)
More typical instant-on: 1000-3000ms (~1-3 seconds), targeting particular vehicle(s)
Constant-on: >3000ms (>3 seconds), not particularly targeting any one vehicle