A standard Zippo will leak/evaporate regardless. A tight new one will probably last a week, a loose older instance or imitation lighter will probably last 3 days or so. You can slow the evaporation by placing a piece of rubber tube over the seam between the lid and body, and it will stay fueled much longer ...
But If you want something that will stay fueled for a very long time, AND can still be made to work a few different fuels, look into a 'peanut lighter'. County Com used to sell them. They use the same whicks and flints as a Zippo, but they have a screw on cap with an O-ring seal. They will stayed fueled up for months or even a year+, are made of stainless steel, and cost less than a Zippo. Win.
The 'Metal Match' is another viable long-term alternative to the Zippo. I own all three (Zippo, Metal Match, and Peanut), and I would never use the Zippo for anything serious when I own the other two.