G'day Mate,
The trick to using Epoxy, is to only mix a small amount at a time, just enough for what you need, the larger the amount mixed, the more heat it produces, because of the chemical reaction, & the faster it goes off. This is true whether it is 5 minute, 15 or 30 minute, if you find that you need a bit more, if you didn't mix enough to start with, just mix another small amount to finish the job.
Also, hot weather, makes it go off quicker too, obviously.
So, for instance, you were glueing 8 pinned hinges into a wing for ailerons, 4 per side, mix enough 30 minute epoxy to use for 8 hinges, you won't need much, & stick the hinges into the wing side first, then let it dry, & mix another amount to use on the aileron side, it is better to mix a bit more, than to have it go off before you can use it.