That depends on the spell. A basic Cure is really only going to fix up scratches and bruises. It makes the pain a little less, too, but not by too much. Cura is the next step up. That one actually heals you continuously! The better you get at it, the longer it works, though it's not that much stronger than Cure. Curaga is the strongest spell. It can't fix broken bones, but if you set a bone it would start healing it. A Curaga can help stop bleeding and start knitting up wounds. It's pretty strong.