I think its easy to forget how huge the Museum is. We stayed for about five hours (90 minutes in the Pompeii exhibition) and still hadn't managed to see everything.
The exhibition itself was well worth a visit (but on a quieter day than Saturday). There was a very naughty statue in one of the rooms that might need censoring around small children but apart from that it would be an amazing family day out.
Having an obsession with Alexander the Great I was a little bit disappointed with the room named after him, and we couldn't get anywhere near the Ancient Egyptian exhibitions.
Note to self - pick a quieter day next time!