How long does dental anesthetic injection last?

I had my 2 wisdom teeth removed. One in bottom right, one upper left. Bottom right one was horizental. Doctor gave me total of 4 anesthesia injection. 1 for left and 3 for right. It s been 8 days, but my bottom right side lip and chin is still numb. Is this bad? Do I ever get my nerve? I went to my dentist yesterday and he said it will wear off? Is it true? Do I not have to worry?