There are two time factors when brewing. First is the how long should it take to brew a coffee, second is the how much time does it actually brew. The fineness of the grind will determine how long should it brew. This means that the finer the grind is, the faster it would take to brew the coffee. Less brew time is better when the grind is finer because more surface area is in contact with the water and the coffee solubles dissolve more quickly. However, as most people brew their coffee, the amount of time, it takes to brew will actually depend on you. This means for non-espresso preparation, you can stick to one grind and pick a brewing time to match. In espresso, the grind fineness also determines the brewing time, but does so in the opposite direction from the time it should brew. The finer the grind, the more the coffee puck resists the flow, and the longer it takes to brew the same amount of espresso. But the finer the grind, the quicker the coffee solubles extract. In other words, there is only one correct grind setting that gets just the correct timing, and even small deviations screws it up, giving you either an over or underextracted espresso. In practice, good baristas will frequently make minute adjustments to the grind to keep it at the sweet spot as beans age, and ambient conditions change. Experience shows that the correct timing for espresso is brewing one ounce singles or two ounce doubles in about 25 to 30 seconds; grind fineness should be selected to produce this volume in this time. Many home grinders only have 10 to 20 settings over the entire range from fine to coarse. This translates to about 2 to 4 settings in the espresso range, which is not enough to get the grind right. An espresso grinder either needs a stepless adjustment, or at least 40 settings over the entire range in order to work well.
In theory, if all the ground coffee had the same particle size, it would all brew at the same rate and you could get a perfect extraction. If the grind size is not uniform, the smaller particles overextract, the larger ones underextract, and the result is less than perfect. Unfortunately, coffee is brittle and shatters as it is ground. So even the best contemporary grinders produce a wide distribution of particle sizes. Moreover, some size variation is required for the mechanics of the espresso puck. If all the particles had the same size, there would be large gaps in the coffee puck, and the pressurized water would gush through. A wide distribution of sizes creates a dense pack that resists the flow and allows proper extraction. This is probably the reason why high grown coffees don't do well as espresso since their fines (smallest, dust like grind particles) create a very acrid taste. Reference: home-barista.com