The time it takes to bring on a full barrage from the first Spotting round depends on several factors in real life.
1) The distance that the guns are away from the target. Usually the bigger the gun the farther away from the battlefield it it.
2)What kind of stress is the Spotter under. If he is under fire his accuracy will usually suffer.The life expectancy of an Arty spotter is very short in a battle. He is a high priority target by the enemy and also by me in CM :-D
3) how good is his LOS. Does he have a clear Los or is it obstructed.
4)The experiance of the spotter and gun crew.
I'm not sure how all of this is modeled in CM but it seems they have taken these factors into account since the time between the spotting rounds(yes it can be more than one) and the barrage varies.
The US army was very adept at putting arty on a target in WW2. They would coordinate attacks so that various guns at different distances would arrive on target at the same time.Try this in CM with 60,82mm morters and 105,s etel. It's tough to do but when you can bring several batteries down on a target at once it is VERY effective.
"No man ever won a war by dieing for his country. He won it by making the other poor sob die for his country" PATTON