It would be a guess on how much glue you would need to build 2 airplanes. I am assuming you are talking about CA glues. Yes they have a shelf life. A lot depends on how warm your building area is, and the humidity. CA's dry faster in warm air and humid air. I buy my CA's in large quantities and keep it in a plastic bag in the refrigerator. I use a 2 oz bottle to build with. When I am finished building for the day, and I know it will be awhile before I need the glue again, I put it back in the fridge. Just let it warm to room temperature before using. The cold can retard the drying time. I use a lot of titebond, and normaaly if I keep the bottled sealed, it will last quite awhile. Make sue you use good ventilation when working with any kind of glue especially CA's. I keep a small fan blowing on my face. You do not want to breathe or let the fumes get in your eyes.