It's been my experience that Great Blue Heron will generally take flight if they catch sight of you 100 yards off. Interestingly enough, it seems it's usually when you stop walking that they take notice.
Yesterday at the Vischer Ferry Nature Preserve, I had the unusual pleasure of meeting a heron that didn't seem too fazed by my presence. It was very near the main parking lot, which was a good thing because the mosquitos were ridiculous.
I saw this heron and stopped, probably only 20 yards away. For the next half hour or so I was able to change lenses, move the tripod around, swat mosquitos, all in plain view of the bird.
At one point, it walked out into the water, toward me (I had to take off the teleconverter to fit it in the frame) and into a spot of sunlight filtering through the trees.
You can see these photos and a couple more in the Birds and Insects Gallery of the zenfolio site at http://trulandphoto.zenfolio.com/birdsandinsects.