It depends on the topic. If someone has posted their work, their latest "find", is asking for opinions, etc., and there are pages and pages of posts, I may not read them all, just leave my compliments.
However if it is to do with a technique, a question, or the like, I'll read all the posts to make sure I understand thoroughly, and see what others have already contributed before posting a response.
For example, this thread is asking for a response regarding what I do, so I didn't feel a need to read all the responses before posting.
Wendy