Bleach is 6% sodium hypochlorite, so that is the same stuff, just a higher concentration. Thus, you could be able to use approximately half as much to achieve the same affect -- assuming it really is 12%. Sometimes the higher concentrated stuff degrades faster so it may not actually be that potent.

As far as how often you'll have to add it, you'll just have to test and learn the pattern of your pool. For now, I'd test every evening and add enough to get back to about 6ppm. Then test the next evening and add enough to get back up to 6ppm. With a CYA of 40, you don't want it to drop below 3ppm.