It does harden when exposed to air. BUT I don't know how long it takes. When I have used my Wilton brand, taking the lid off, there is some "crust" around the lid. I work PT at a DQ where we make "chillabration" cakes and we use it for the designs, etc. and the bags we put the gel in with different colors gets dried up where we twist and tie with a rubber band. Also, before we color a cake or write a name, etc, I run the tip under hot water and squeeze out so it will work. If it gets too hard, I unscrew the coupler and clean the tip.
I would imagine if you did some testing with it, you could get your own results.