How long can you leave primer exposed?

I am planning on sanding down and primering my CRX. The hood is faded badly and some of the paint has chipped off from rocks and stuff. Just little chips barely noticable. But I dont want it to start rusting on me. Also the right fender is black while the rest of the car is silver. I had to replace it cause the previous owner dented it pretty bad. So I wanted to just sand the entire car down and primer it so it would atleast match. I was wondering how long the primer would last? Being rained on and etc.

well, im not sure but there is a primer crx that i see all the time, and i've seen it for a year straight with primer, and he still hasn't painted it. i've seen it up close and i don't see any rust or anything so it should last pretty long. It also rains alot here so the primer probably protects it from rusting.