The M phase is the shortest phase of a cell cycle. Also known as the mitotic phase, it occurs over a period of 2 hours. 

The cell cycle consists of 4 main phases, which occur over a period of 24 hours. 

The G1 phase is the longest, with a duration of about 11 hours. 

The S phase is the second longest. It lasts for about 7 hours. 

The G2 phase lasts for 4 hours. 

The M phase is the shortest, with a duration of only about 2 hours.