Mixing most epoxies for a minute is more than adequate. I normally don't mix in a cup, I prefer to mix on a flat surface where I can mix the components folding them into together by scraping them up with the edge of my mixing tool, then spreading the material out again ensures a good mix.

A good mixing tool is a artists pallet knife sold in art stores. I like one about 1/4" in width that is flexible with about a 4" blade. I use the same knife to apply the epoxy it allows for pretty detailed application even in most tight areas.