How long do you have to wait to read a tb test. TB skin tests should be read between 48 and 72 hours after the injection when the size of the induration is maximal. Tests read after 72 hours tend to underestimate the size of the induration. ChaCha on! [ Source: http://www.chacha.com/question/how-long-do-you-have-to-wait-to-read-a-tb-test ]