i wait until the entire skeletal structure has encrusted with various algaes.. then I call it in. I have one "colony" that RTN'ed down to two polyps and its immediate surrounding tissue. There's probably no more than 5mm worth of tissue around the polyps and it has been like this for a month now
I guess it is in a perfect zen balance or something. I would frag it but the two polyps aren't even on the tips, they are in the middle of one of the branches. I wasn't sure what to do about it so I just left it.