I've got a job interview next week and the interviewer asked my colleagues this question. I didn't find the answer online, I personally don't think that there is a certain duration for a wind shear because it's a sudden change in wind direction and speed. The question is annoying actually and I really want to know.
They want to know how deep is your general understanding of W/S. My answer would be, "What kind of wind shear are you asking about? Microburst from a thunderstorm cell? Descending through a nocturnal jet (large change in wind velocity at a few hundred feet at night)? Wind shadow shear just above the surface?" (it's not unusual in a jet to hit a little 5 to 10 knot shear at between 100-200 ft when landing, just from the effect of trees and buildings slowing down the surface layer of air) and go from there, based on the research I've down beforehand.
There is no time limit. Could be 5 seconds. Could be half a minute, the time it takes to cross a mile wide microburst at 120 kt. Whatever. The key thing is to do some googling and read some articles on windshear so you can sound like you've taken the trouble to learn about the topic.
So don't just look up that specific answer. Become knowledgeable on the phenomenon itself. That's what they want to see.
At some locations, wind shear can be a nearly permanent condition, such as on the lee side of some mountain ridges affected by steady winds. On the California coast during the onset of so-called Santa Ana conditions in the fall or winter season, there can be wind shear at the top of the marine boundary layer (typically around 1000' altitude) that lasts for a day or longer before the higher winds reach the surface. Microbursts, on the other hand, are generally short-lived phenomena, typically much less than 15 minutes.