When watching rocket launches, the commentary or status reports often mention that the vehicle is "passing through Max-Q" - how long does the "passing" take? Is the event instantaneous or is there some time interval while the dynamic pressure stays maximal?
"Maximum Q" is technically instantaneous, but less formally it can refer to a period of time where pressures are nearly at maximum.
Here's a graph of dynamic pressure for a simulation of the Saturn V; (gone missing; archive.org link) most launchers will have an essentially similar curve. You can see that it peaks close to 34kPa, but it's above 95% of that value for a period of at least 10 seconds.
The mission abort criteria for the Saturn-Apollo flights use the term "max Q region" to refer to the period of flight time from 50 seconds to 2 minutes elapsed -- this span of time includes dynamic pressures only 1/4 that of the actual "max Q"!
Max Q takes 21.6 s (according to ST-118 numbers) or 27 s (according to RocketLab numbers) or 12 s (from the test flight of Falcon Heavy).
As the other answers say, the actual peak of dynamic pressure is an instaneous point, but actions taken around this time are often described in terms of Max Q. For example, Space Shuttles would throttle down their main engines for 30 seconds or so while Q peaked. (The SRB propellant grain was also shaped to lower thrust during this time, but that was pretty invisible.) Typical throttle command times for this "thrust bucket" were:
Throttle down from 104.5% to 74% --> 32.8 seconds Mission Elapsed Time
Throttle up from 74% to 104.5% ----> 54.4 seconds Mission Elapsed Time
Numbers are from STS-118, but are representative.