If you mean a flash burn of the eye it can take 2 to 3 days to heal and for the burning sensation to go away.
During that time you should put a cool rag over your eyes to help heal them and keep the eyes cool and you may be temporarily blind for at least a day and then your vision should return.
If the flash burn is on the skin then it may take a week or two weeks for the flash burn to heal and you can put some Aloe Vera gel or other lotion on the skin to help the burn feel better and help it heal.