30 minutes early should suffice. We went during Spring/Easter break last year and it wasn't very crowded at all in the IllumiNations FP+ pen. BUT...there had also been some major thunderstorms that afternoon and early evening, and I think it was even drizzling a bit while we waited for IllumiNations to begin. So that may have affected the crowd, I don't know. I was able to get that FP+ the day before, so I don't think it was really all that in demand. Same for Wishes FP+ (back when they had them). I was checking the app the night before and got one, so we did it. Same as IllumiNations, it wasn't all that crowded and we sat stretched out on the ground.
For IllumiNations we hung at the back/top of the slope until the show started and then moved forward. We weren't front row, but close enough. It was an amazing view!