We went a couple of years ago and stayed at a CL not too far away so that we were in close proximity to the NEC. If you are able, as Graham has suggested, I would go for the two days as it is such a massive show.
If you are in for a new van there is so much to see and so much to take in. We were in and out of so many caravans that after a while we couldn't remember the details of all the vans we had a look in. Therefore I would recommend taking a pad and jotting the details of each caravan that you view.
Then on the second day revisit your favourite choices having re-grouped overnight with your list and a glass or two of wine!!