You keep using the word "estimate." To me that means just that and the final price can be higher, or lower. I think you can charge her more. You have let her know that it is taking longer than originally quoted. I don't know how much you are over the original estimate, but I think adding 10-15% would not be out of line. In the future, I think I would give a written estimate in which you spell out that this is an estimate and the final price may be different and then it is signed by both parties.