Tente isto:
SELECT SEC_TO_TIME(AVG(TIME_TO_SEC(`login`))) FROM Table1;
Dados de teste:
CREATE TABLE `login` (duration TIME NOT NULL);
INSERT INTO `login` (duration) VALUES
('00:00:20'),
('00:01:10'),
('00:20:15'),
('00:06:50');
Resultado:
00:07:09
