Most mouth sores go away on their own without treatment within 10 to 14 days.
The mouth sore should begin healing within 4 to 7 days and then after 10 to 14 days the mouth sore should be fully healed.
If the mouth sore is inside your mouth you can gargle with some apple cider vinegar which helps to kill the bacteria causing the mouth sore.
You can also gargle with warm salt water as well to heal mouth sores or gargle with some hydrogen peroxide.
I use apple cider vinegar to gargle with to heal mouth sores such as mouth ulcers because it kills the bacteria and I notice it heals the mouth sore within 4 days versus taking 10 to 14 days to heal.