I saw Harmonious in early-mid May and I secured my spot in between Port of Entry and Disney Traders at 7 pm when the show started at 9, so 2 hours ahead of time. If you can believe it, there were people already sitting up against the fence before we even got there. Now you don't need to camp out 2 hours in advance - my friend wanted a front row view so that's what we had to do to get it. However that entire area was 100% full by 8 pm.
This was our view of the show just in case you're wondering:
There is an empty area in front of the viewing area but Cast Members block that off to guests so you are guaranteed to have no one in front of you.
We saw it from Tom Bricker’s “Japan spot” both times we watched it, during the busiest Spring Break week in March. We were able to get a great spot 10-15 minutes before showtime, and could have walked up as it started. It’s not the straight-on view above, but it is a clear view of both the Stargate and Tacos.