I can't give any numbers either, but if you want a baseline: gasoline evaporates faster than water.
The rate will probably depend on temperature, humidity, amount of air in the gas tank, surface area of gasoline exposed (i.e. tank geometry)...
I suppose plugging the line may work, but you'll have to deal with increased pressure. If your tank doesn't generate pressure when the line is plugged, that means it is being vented somewhere else.
Modern cars use activated charcoal canisters to trap gasoline as hot air is expelled. I would look around a junkyard and retrofit one of those canisters. I can't imagine it costing more than $10 in parts. There are schematics online showing different types of setups.
Lastly, preventing gas from evaporating helps lower pollution a bit by decreasing HC's and VOC's. I like my skies blue and lungs healthy...
- LostCause