The phrase, “in the blink of an eye,” is an idiom in the English language that has a somewhat literal meaning; the phrased is used for when a very short period of time passes. Speakers and writers use “in the blink of an eye” to refer to anything that seems to have happened quickly. In actuality, though, English speakers often use this phrase to apply to things that happened over a long period of time, but which appeared to happen quickly in retrospect.
It’s easy to see how this saying relates to a very quick time frame. The fact is that it takes only a fraction of a second for someone to blink his or her eyes. A common synonym would be "instantly."
Another idiomatic phrase that is used in place of “in the blink of an eye” is a bit more colorful. English speakers often use the phrase “in two shakes of a lamb’s tail” in place of the more literal reference. Here, listeners assume that it does not take a lamb very long to shake its tail.
Other synonyms consist of single words. For example, someone who might be talking about a very short period of time might say “in a jiffy” or “in a nanosecond.” The phrase “coming right up” is used in similar situations where someone is waiting for something.
It’s important to note that the phrase, “in the blink of an eye,” is rarely used to communicate to someone who is waiting for something. In contrast to some of the above phrases, this phrase is more commonly used in reflection, nostalgia, or in a similar situation to express a feeling about how quickly something has happened. For example, “the kids grew up in the blink of an eye” is a very common application of this phrase. The children in question have in fact taken years to grow up, but the speaker is using the idiom to indicate how quickly the long period of time seemed to pass. This sort of use demonstrates how English speakers and writers most often use “in the blink of an eye” as a fancier way to say that they have perceived time to have passed quickly.