I am planning on making some chili powder this weekend using ancho, pasilla, and guajillo peppers. I got a nice big bag of each recently and have some questions about chili powder. The second part of this question, about [tweaking the preparation itself, is here][1].

The first concern I have is storage lifetime. I keep reading that it should stay potent for about 6 months, in an airtight container. I have also read that I can mill/blend the peppers and freeze the powder for 6 months and then use. How does storage work?

 - Do these 6 months stack on top of each other; or is it six months potency one way or the other? 
 - Is there an alternative preparation of/method for using the milled chili (powder) that would allow me to get a better shelf life? (My main concern is not having spices that last forever; rather it is to have them remain potent for a reasonable period.)
 - Does the "6 month potency" begin once the chili powder is prepared, milled, or when the peppers are dried (or some other time)?

  [1]: https://cooking.stackexchange.com/q/13050/15