Personally, I could never blatantly self promote through commenting; it just doesn't sit right with me. When I do comment, out of genuinely feeling compelled to, I try to keep it between 2 - 4 sentences; pointing out something I liked from said video, or a commonality I've noticed in their content. This has proven extremely fruitful, my subscriber and view rate has greatly improved, and have become friends with some of the channels I regularly comment on! That's a win-win in my books!