It's like 'a pinch' of salt in some respects - being that we all have different sized fingers, we all get a different amount - but its always small.
With that said, a dram, I gather, is somewhere between 1/8th and 1/16th of an ounce.
A "fifth" is a fifth of a gallon. Since a gallon is 128 fluid ounces, a fifth is 128/5, or 25.6 fluid ounces. I'm not sure anyone still uses actual fifth bottles, though; I think nowadays they're...
3/4 ounces
1 ounce = 8 drams. To convert a value in ounces to drams, multiply by 8.
256 drams
There are 16 drams in a ounce