actually what i have heard is that lemon juice should not be directly be applied to the face instead it should have something mixed with it like milk or rose water and that is exactly what i do evry night!
It depends on what you want to use it for. If you don't have sensitive skin, you can probably use just plain lemon juice as an exfoliant. Rince it off after about 15 minutes. However to use it to maintain your skin's PH balance, mix it with water first, then just leave it on, or if it stings or burns, rinse it off with just water, no cleanser.