probably about an hour before - but you will see some people getting their spot some time before that - and some people who just stroll up at the last minute and push right in to the rope
About an hour before should give you some good spots to chose from - unless it is a very busy day.
I find a spot one hour before the parade starts. I like to sit in front of the candy store on Main Street. That way son doesn't have to go far when we need our candy apple or caramel apple fix.