How long should numbness last after wisdom teeth extraction?

2 versions: If you are talking about the time you will be numb from the local anesthesic, it generally lasts for a few hours. If you are talking about numbness due to trauma to the inferior alveolar and/or lingual nerve it's a different story. These parasthesia's can last from a few days up to 12-18 months. There are some cases where the numbness can be permanent. Discuss your specific situation with your dds.

How severe?: I have first hand experienced getting a numbness in my tongue when my wisdom tooth was extracted at age 17. Half of my tongue was numb for few years and now after 23 years it is much better still not back to normal. It all depends on how severely the nerve is damaged. Sometimes it is very transient lasting from days to months and sometimes for many years. I hope no one ever experiences it!