I likewise had this problem. I have 2 columns fst_response, and time_in. I wanted to calculate the average response time.
Because avg() does not work with mysql's time format you need to convert to and from seconds:

mysql> select sec_to_time(avg(time_to_sec(timediff(fst_response,time_in)))) as t1 from cases;

+----------+
| t1       |
+----------+
| 00:23:03 |
+----------+
1 row in set (0.00 sec)
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