One word to describe the Hermitage would be huge. The place is so big that I spent an entire morning there and almost covered one floor.
The rooms themselves are amazing pieces of art. Each room is decorated differently and has amazing frescoes on walls and ceilings. The entrance hall to the Hermitage takes your breath away when you enter it. There are golden chandeliers everywhere, and the walls are covered in golden decorations. There are red rooms, blue rooms, green rooms, and every other colour. The beautiful paintings on the walls and ceilings could hold your attention for hours on end.
Then comes the art. There are apparently enough pieces on art in Hermitage that it would take you years to look at them all. Well, I definitely experienced that. I spent the whole morning there and looked around almost one floor. There are paintings, sculptures, jewellery, etc., from all different periods of history all over this museum. There are maps available for anyone who needs to find their way around, and I strongly recommend using these if there are pieces of art you really want to see.
If like me you just want to have a look at lots of different types of art, then just having a wander around is a very nice way to spend some time, and occasionally you can listen in to guides giving other people a tour around the art.