[Tricon]

I feel comfortable with defense 15 at the borders. Usually I get most of my provinces to Def 10-15, no matter which country I play.
However, I don't see them as a real defensive force. Just as nuisances. I usually have a fast moving small-to-medium army as an emergency reaction force. The longer the game is the more of these forces I raise. The size is often 1/3 to 1/2 the size of an attacking army.... just big enough to win a defensive battle if combined with defense and maybe a few hastily recruited forces that were on my way.

Everything else depends on the country I or my enemies play. And circumstance, of course.