@Carrie I think going at 10 am was a good idea! Our entry time was 3 pm on a Friday (gallery open til 9 pm) and it had become so crowded that entry times were not being observed, as far as we could tell. Or it may have been that everyone was there to see the exhibit because when we went out into the main galleries, they were virtually empty! What a joy to peacefully wander around, admiring my most famous paintings!
I would add that the National Gallery provides an excellent and comprehensive free guide to the Van Gogh exhibit as you enter. The audio guide really isn't necessary, in my opinion, unless one needs/prefers one.