Just a rough estimate here. (assuming a ballpark of 20 drops per ml and 3 drops dripped gets me a round figure of 5 solid hits.)
1ml / 20 drops = .05 ml per drop
.05ml per drop x 3 drops = .15ml per full atty
.15ml / 5 hits = .03ml per hit.
Many of these variables are assumed. Drop size is not a reliable measurement. The best way to figure it out would be to do what venkman suggested. carefully keep track of how many mls you are using and how many drags you are getting. The larger the sample of data you take the more accurate your final results will be. As a side bonus, if you keep track of your data for several days or more you can start to see how many mls you go through per day as well.
Venkman: is that old-school SMS ghost busters you have as your avatar?