You should always follow your dentist's instructions, since they'll know your history. For an average individual, you're probably going to have to bite on a piece of gauze for 20 minutes. Then, you take the gauze out of your mouth and replace it with a fresh piece of gauze. After biting on that for 20 minutes, check to see if there's blood on the gauze. If it isn't really bloody, you're done. If the second piece of gauze is bloody, you need to keep going. If your gums start bleeding at home, take a wet tea bag and stick it in your mouth and then call your dentist if it doesn't go away soon.