A good rule of thumb is that the corners of the mouth extend to the middle of the eyes (if you draw a line down the face and the eyes are looking straight ahead, the corners of the mouth should line up with the pupils.) the upper lip should be longer than the lower.
(I hope this helps, my art teacher did a lesson on this and i found it pretty helpful).
(I hope this helps, my art teacher did a lesson on this and i found it pretty helpful).