I combine some of the contents of two (2) Rema Tip Top TT02 kits into one box. That gives me one "in use" tube of vulcanizing fluid, and another as-yet-unpunctured one as a backup.

I understand that they won't last forever, but that beats one of the likely failure modes. That said, I've had an "in use" tube last quite a long time; perhaps just to spite me.