50% CDR will make him use his skill twice as often. That's not a specific number of seconds, but it's still useful information. For many heroes, 50% CDR makes them almost twice as powerful (Ninja and Thief are the best examples I know of). If you run the game slowly in campaign, you can count the number of seconds it takes to fire each skill. This will give you a rough estimate of the effectiveness of CDR.
However, they can't use non-white skills while using white skills, so if you have a hero (like Wee Witch) who just constantly uses whites, CDR isn't useful. You also don't want it on heroes like Bard or Archer, who depend on basic attacks for their damage.
CDR on Faith Healer wouldn't be very useful. Evangelize is great at lower levels, but it's affected by Tenacity, making it all but worthless these days without a ridiculous amount of Improved Disables.