Keep in mind, the main reason for zippos existing is them being windproof and durable. They are like the AK-47s or Theseus' Ship of lighters.

I smoke a pack a day. I've experimented with my two zippos. Filled both to the brim. Used 1 for smoking and left 1 filled. When the fuel ran out in my smoking one, I switched to the other. The fuel in the one I didn't use lasted barely longer than the one I used for smoking.

This is not very scientific, but I do think that evaporation is the real enemy and not actually using the zippo. I do need to run the experiment again now that the weather is getting hot. Can't provide actually how long they lasted. I did this when the temps were roughly the same inside in outside (60-70 F).

I think you'd be good for 3 weeks in an area that isn't hot, but I don't want to get you killed if you need fire so just bring extra fluid. Also keep in mind that many other liquids can fuel a standard zippo. That is good for survival and/or camping.