Seems like it would be hard (no pun intended) to measure. I've experienced major differences because of variables like...
age
how long since last cum
effect of (or absence of) party substances or aphrodisiacs (Maca, Ginseng, etc.)
how long working it up (e.g. edging for 3 hrs vs. a 3 minute rubout)
how turned on - e.g., is the setting something new/different/wildly exciting or is it more or less routine)
I've seen enough porn to know that dick size doesn't seem to matter - seen dribblers and shooters of all sizes.
Then again there's the Peter North phenomemon, where some guys squirt like fire hoses apparently regardless of other variables. If somebody ever does a study, I'd like to be a lab assistant.