Right, but what I'm asking is how often you need to cast defensively, in general, not how often you make the roll.I had it, and I barely ever failed to cast defensively... I think I had a +23 concentration roll by the end of the game, and the highest DC I ever had to make was DC 23 to cast a 4th-level spell.
I'm thinking that at least levels 1-10, I can avoid the need to cast defensively most of the time. And if I do need to do so, I can use Spell Combat to dump a +5 circumstance bonus on it. At least in theory.