How would you calculate the air that a pneumatic cylinder consumes?

Context: I am a technical student who is interested in mechatronics and PLCs.

Not sure if this is the right place to ask, but figured that people in automation are somewhat familiar with pneumatics.

I was under the assumption that I simply needed to take the volume of the cylinder chambers (both in and out), mupltiply it by the absolute pressure and the number of times the cylinder is moving forth to back.

So somewhat like this: Q = (V1 + V2) * p * n

But my mentor has told me that there are cases where a cylinder isn't continuously under pressure, where it only get air for a split second, and the air's expansion causes the cylinder to move forward, so in these cases my calculations are incorrect.

My question is, how common is this? Never heard of it.

Thanks in advance!