If I'm going to comment on it, then "All of the posts". The only exception to this is if it's over about 30 pages, and even then, if it's good enough to comment on and I won't read it all the way, I'll read a huge chunk of the pages, comment, then go back to read the pages I've missed.
If it's not an interesting enough topic for me to read the responses, then generally I won't post a comment myself.
Of course, there has been the occasional exception to this rule, but that's to be expected, surely?
