My answer to the original question is, "when you've tried everything you can think of, the school/district has demonstrated that it isn't going to do anything significantly different, and/or the harm to my child reaches the point where no further patience is warranted."
Yeah, that's a good summary. For us, we stop when we don't see any real possibility of change. But that doesn't mean that you can't start advocacy again when the time is right. Our advocacy for DS basically never worked. We told him to just have fun at school and we will cover academics at home. Until when he got into a magnet high school with strong academics and some true peers. But for DD, things changed dramatically when 1) there was a new principal at her school who has a specialty in GT education; 2) online courses became standard offering in the district; and 3) we were going to leave our elementary school for good and no need to worry about burning bridges.
