Lots of 'If's' and Butts' here.
Depends on the size of your roof and the rainfall in your part of the world. ie; If you get x" of rain in a 12 sq" container over X days, then multiply this into the sq" area of your roof. That will give you x" depth of rain. (There is 1 (UK) fluid oz per 1.734 cubic inch -or- 1 UK gallon to 0.005 cubic metre.)
Now, doesn't that make you feel better?