Go to airsoftmechanics.com and read up on gear ratios there. RiotSC has written up whole articles on gear ratios, calculations on theoretical ROF and even designed a dual sector gear (shoots twice with each rotation...neat).
Another way to look at it is that the Modify High Torque gear set has a ratio of 21.6:1 and their High Speed gear set a ratio of 16.32:1. A "normal" ratio is somewhere inbetween there.
Another way to find out more is to read some of the stickied upgrade guides on this site. There's talk about the different gear sets...more general stuff vs. specific numbers.
A point to note is that unless you're going to get into the math of it (and I haven't seen that on this site...try airsoftmechanics), it's not worth locking into a specific set of numbers.
Do some more searching yourself...the info is right there in front of you.