Very very early if you want a good spot, but to be honest, unless you can hold your bladder for an extremely long time, or are with friends who can hold your space, you will more than likely lose your original spot, given the race doesnt start until 4 or 5pm local time. I got there at 12pm and the turn 2 GA banking was 100% covered in people.
Its almost worth getting a catheter installed just for the day lol