Its a 5.30pm start so I suggest you arrive by about 5pm in order to find your way to your seat. You won't need to hang around outside as there are vast areas inside the O2 and it takes 15-20 minutes to get from the entrance to the seat. There are shops and restaurants and plenty of loos, so take you time and enter leisurely. Be sure to get to your seat by 5.30pm.
I don't know the finish time but i do know that award ceremonies are long evenings (by the time each of the winners have walked up on to the stage and given their speech) my guess is you'll be there until about midnight. But maybe others know from first hand experience.