When I learned to be a Barista in Aus back in the day they said a 'good' shot pour was 25s-30s to get a 30ml shot. If it was less than this then it was said that we hadnt extracted it properly and longer extracted too much although your taste comes into play also a bit. Either long or short pour didn't taste good, like bitter taste. Between 25s-30s taste was said to be maximised. If you like it bitter then this may not apply I suppose but most people dont in my experience although its v common to get a bitter coffee in many cafes I've been to.

If you wanted to play with this method you could... pour a shot as you normally do and time from start to finish to see how long it takes. If not in this range you could adjust either the grind or the hardness of the 'tamp' to change the time it takes to pour the 30ml shot.

By making a smaller (finer) grind your make the coffee particles smaller and hence the water takes longer to pass through the coffee. Larger grind would make it faster for the water to go through the coffee. If you are grinding your coffee yourself this is adjusted at the grinder. If not then you cant adjust but could play with the 'tamp' but adjusting the grind is much better.

The tamp is after you've added the coffee to the basket its often 'pressed' or tamped down with a flat round metal object called a 'tamper'. Pressing it also makes it more compact and harder/slower for the water to pass through. Mostly the size of the grind is used but this also plays a smaller part.

They went into more detail about humidity and how it changes over time of day, where the machine is etc and the humidity of the day could mean that a shot poured in 25s-30s in the morning could produce a shot pour outside of this range in the afternoon. As such they used to time each shot pour to notice if things had changed. If so they adjust grind again and tamp to get it back into range. If it was longer than 30s youd adjust grind larger to speed it up and lower the pour time. If it was shorter youd adjust the grind smaller to slow down the shot and get it back into range. 

There ya go thats more or less a full day course in espresso how I learned it back in Aus. This was in like 2010 or so.