I can only tell you what I've read:

According to one of the manufacturers (ZAP, I think - Tiano?), epoxy does not go bad. What it does do, however, is get thick (crystalize)? If it does get thick, heating it in the microwave for 10 seconds or so will get it back to a liquid state.

I also understand that 30 minute is stronger than 5 minute - mainly because it soaks further into the surfaces being glued.

RCPAUL