The epoxy primer should have a longer shelf life than the catalyst but kept sealed up should last 10 months.
To me the job dictates the type of paint. If you were painting a car or restoring a tractor which takes a great deal of preparation, then I would say you should use the epoxy primer. For a equipment trailer I would just use an alkyd primer and paint. Its not as durable but should last 3-4 years and you can just shoot it again in an afternoon if you don't let it completely fail. A lot depends on the color. Paints with red in it fades a lot faster than other colors. I painted a trailer gray in summer 2009 and it still looks good. Only places where I have damaged it with chains are rusting.