Like Melanie, I would say like 2 hours (maybe even more) so that you really have time to get ready, to settle, and to be calm. A show already generates a lot of stress because of the pressure and because there are so many things to think about, so if you take quite some time, you will be sure not increase your stress because you will have time to see things coming.
I usually have 1-2 hours!! If you are taking your own horse it’s good to be early to get them used to everything and have time to chill before actually showing but if you’re doing IEA you can be there as late as you want but most riders get there early morning to help out with the horses your barn is taking to the show. You also get your back number in the morning and your trainer goes into a meeting to draw the horses so I would personally be there earlier to be able to get settled and have a chance to read my horses description and maybe go see him etc