If the topic is such that I really feel I want my input added I do no matter how many pages it is. If it is a picture of a quilt that truly takes my breath away and I absolutely love I will also post. I don't care if the topic is 20 pages long. Just because you have nothing to add that is unique or different doesn't mean your comment isn't truly appreciated by the OP. I know I truly appreciate every comment made on one of my topics. I have never had one go more than 10 pages but if I did I would be thrilled! Wouldn't YOU be thrilled if you posted a picture of a quilt you did that got 20 pages of responses, even if they all said "wonderful", "beautiful", etc?? Rather than thinking you don't have anything to say that hasn't already been said, or you are just trying to keep your minimum post activity up to keep your status (a poor reason to participate IMHO) why don't you think about how thrilled the OP will be with all of the comments?
I don't post on every single quilt I see. I pretty much only do it on ones that move me in some way.