My car has 182,000 on it now. I flushed the lines and master expecting a bunch of rusty, cruddy-looking fluid to come out. The fluid that came out of the front calipers looked just fine, though it may have had some invisible moisture in it. The very first bit of fluid that came out of the rear wheel cylinders was kinda dirty looking, but the rest looked okay. I ran about 16-20 ounces of fluid through so I am pretty sure I got it flushed completely.
I wasn't expecting there to be a big difference in the way the brake pedal felt before the flush and after, and...there wasn't. When to do it? I am sure there is a recommendation in the manual, but I'd say at 100k might be a reasonable plan.