I'd recon 3 nights as a minimum, and if you want to have a reasonable look around, add a day for chilin, and another day to go back to the falls just to make sure.
The camp ground at Adel's is pretty big, and is not designated sites (set up where you like). It was late July when we were there and there was plenty of room, doubt that you will
need to book, but if you know when you are arriving, it can't hurt.
It's about a 15min drive to the gorge from Adel's Grove, and the camp ground at the National Park has limited sites with basic facilities, and I'd recommend booking if you wanted to camp there. But my choice would be Adel's Grove. The facilities are much nicer, and there is a bar and restaurant if you feel like treating yourself to a night out.
There is no TV, Internet, or Mobile reception at Adel's Grove (the bar staff are very appreciative if you give them a recent newspaper

), but there is a hill about half way between Adel's Grove and the Nat Park where you can get mobile reception if you poke your tongue the right direction.