It is probably possible to see it in one day, but two days is much better. Of course, we watched every show and did most of the attractions, some twice, with DS 7 and DS 4 so our pace wasn't break neck by any means.
We were there yesterday and I'll second the need for 2 days, it's been 15 + years since I've been there and the place exceeded my expectations. What a fun place.
I'd say two days too. We went for four days over Spring Break and never tired of it! One day can be done, but it's pretty tiring to race from one show time to the other, usually on separate sides of the park.