For me, actually, I just comment when I feel the need to, you know? If it's a close friend, I comment. If the artist asked for any critques or thoughts, I comment. If I think this newer artist needs some love, I comment. Heck, if the artwork is awesome and I just have to say that, I comment! Its amazing if people reply, but all I'm hoping is that the artist at least saw what I had to say.