4 month sleep regression

How long does the 4 month sleep regression stage last? My 4 month old slept through the night since birth so we can't really complain. The past two weeks he has been waking 1-3 times a night. Last night he has been awake since before 5am and no matter what would not go back to sleep. Its now noon and he's still fighting sleep but he is exhausted. How long does this stage last? Is there anything we can do to help him sleep longer and get back to his old routine?