I just use it until I feel like picking up a new gallon or somebody throws it out accidentally for me, usually every few months or so. I used to worry about bacterial contamination in my distilled water and bought a new gallon every week or so, it didn't cost that much, but I realized there's no food for the bacteria in distilled water, even if it got contaminated, as long as you kept it closed, the bacteria/mold/etc would not multiply enough to contaminate anything. Also, as stated before, if you put a "touch" of everclear with distilled water, it's not gonna do much to prevent microbial growth because it needs to be a decent <12% solution in order begin to inhibit microbial growth and even then the alcohol evaporates faster than the water and your solution gets weaker and weaker. Propylene glycol is better suited for the job, you can purchase the premixed solution or request it at the drugstore and mix it yourself up a 50/50 solution.