Depends on what it is and what I'm trying to get out of it. I usually read at least the first page and the last couple of pages. If it is a gargantuan resource thread, I might read more or less. Hard to get through those though so I understand why people just come in and post a question that was answered 15 times.
Also depends on how many pages. 2-3 pages I'll read or at least skim every post. More than that and I can't be bothered unless it's a topic I'm really interested in. I'll read the first page and maybe the latest page or two.
Depending on the topic, I tend to avoid new threads with lots of pages as usually the question has been answered already and is now simply being beat to death.
Controversial threads I read from start to finish, no matter how long. Interesting to read people’s personal thoughts on politics, religion, etc. I usually don’t comment on those topics...... Just like chasing fire trucks and ambulances, I guess.
All other stuff I read first post or two and if it sparks my interest, usually I read all. If not, I will go no further