It depends on the fish' immune system and general health, but most scrapes and fin damage will heal in a few weeks. Just as Neosporin and the like promote healing in human tissues, melafix and pimafix seem to promote healing in fish.
The key is to keep the water pristine - no amount of medication will compensate for dirty water. In addition to using melafix and/or pimafix, I would recommend doing 10 - 20% water changes DAILY until the wound is pretty much healed.
Your question about fish sensitivity to melafix and pimafix is a good one that alot of people don't think to ask. The answer is yes, anabantoids (gouramis and bettas) seem to be sensitive to the plant extracts in both meds. It apparently irritates their labrynth organs. If you have anabantoids present, you should use melafix at 1/5 the normal strength.