Dwell is the length of time the solenoid is powered for.
A dwell of 3ms does not mean the bolt will be forward for 3ms, or that the valve will be open for 3ms. There are many other factors involved, such as the response time of the pneumatics of the gun.
A longer dwell does not mean the gun will be softer on paint, its not that simple.
Too low of a dwell value can mean that you require more pressure to acheive the same velocity.
Too much dwell can cause your gun to be loud and inefficient.
In any given setup there is an optimum dwell time that gives you the best efficiency and concistency.