Well rub consists of dry spices That all have a really long shelf life!
Have you ever seen a exp date on paprika? Caking will probably be your only issue if you store it for a long time. I grantee you'll use it before you have any problems. I make my rub in large quantity's being I use it allot.
Keep all species in air tight containers, and away from any heat. Air and heat will diminish shelf life, otherwise it's good for over a month with ease. Enjoy the trial and error, there's a light at the end of the tunnel!