I would not worry about shelf life for the Eternabond Tape you are going to carry for emergency. Chances are, whatever you apply in an emergency, (with inadequate cleaning and prep) is going to be replaced a short time later with a proper repair.
When I applied my Eternabond a few years ago, I also sealed the tape edges with caulk. At that time I used painters tape to give a neat edge line and applied enough caulk to allow any expansion or contraction flex. I check my f/g roof regularly and have not had any problem with the Eternabond that was applied to the properly prepared surface. If the surface is not properly prepared, the tape will bubble and will be easy to spot on inspection. Just cut that out, clean, and reapply.
I carry two sealed packages of 4"x36" for emergency.