MNS is correct; 1.5 to 2 hours.
IMHO, 2 hours is to be preferred. That way there's plenty of time for pregame warm-ups and the postgame shootout. 1.5 hours is enough for the game itself.
Depending on cost and timeslot constraints, it may be necessary to settle for 1.5 hours for the whole event. That's not an insurmountable problem. But if you go that route, it's best to be a little more organized upfront. Have some tentative teams set before the icetime starts, make sure everyone hits the ice on time, etc.
BTW, the postgame shootout is a tradition that has been consistently maintained, regardless of the score in the main game. Everyone on both teams shoots, even if the numbers aren't equal. Some years everyone has gotten two shots.