How long does the epoxy hold for?
It depends on the epoxy (there are probably 100's of different ones.
It depends on the surface prep.
It depends on the difference in the coefficient of expansion of the two materials that are being epoxied together, and the temperature range that the materials will be subject to.
It depends on the magnitude and type of loads the joint will subjected to. Tension and compressive loads are best. Torsional or cantilever, not so hot.
Find a real source of stand offs. Look for something that has an outside hex shape that is internally threaded. That way you just ave to drill a hole in the case the right diameter to run a screw in from behind to hold the standoff to the case. Adhesives are very useful for many things, this isn't one of them.