You can 'see the show' in a couple of hours.
By that, I mean walk round all the show gardens (there are not that many), and have a stroll round the stalls selling plants, and other products.
If you want to spend a long time browsing through all the plants at a lot of stands, or watching demonstrations in the lecture theatre, then maybe a day.
First thing in the morning you can walk round the show a lot easier than later in the day, when the show usuually becomes more crowded.
You won't find much that is new to look at on a second day, in my opinion.