I think most threads become tiresome after about 30 posts. By that time, most of the good points have been made, many postings become redundant, petty arguments break out, and the shear volume becomes stupifying and I personally dont have much time to read on and on.
What say you? Do you check to see how many posts are listed for an otherwise interesting thread, and think “hell this one already has 49 repsonses…I think I have to mow the lawn”
Just the opposite actually. If I see a thread that’s more than 100 posts I’ll be a hell of a lot more likely to open it because a lively discussion is underway. Some issues are too complex to have been throughly examined with 30 posts.
Around 150+ posts even if the thread looks interesting I often won’t bother opening because I just don’t have the willpower to read through that many posts just to jump in to the discussion.
I’m usually reluctant to join a thread if it has run to a third page; if I was already involved though, I’ll go the distance.
Yes, many debates on page 3+ consist of people just restating their arguments over and over, but it’s not always the case (chiefly because, unlike me, new contributors join the thread).
Thread length isn’t that important factor for me, the thread title is the biggest hook. I will also look at a thread if the last contributer is someone whose posts I often find edifying/informative/funny/annoying.
That said, if a thread is on a topic of little apparent interest but has run to six pages, I will have a look just to see what’s going on. The recent GQ thread on removing a broken-off lightbulb from its socket is a good example.
I’ve never read all 30 posts of a long thread. Usually a few will do.
And I’ve never felt the need to add to a long thread just because I have a favorite answer.
“What’s your favorite city?” Of course I have one, but then so does everyone else.
“Would you cross a picket line if you needed the scab work to eat?” Tough question, but all the viable answers are taken in the first few posts.
I have to disagree. I recall the “Choose who gets the only parachute on a plane about to crash” thread.
True, after the first page, most moral points were discussed, but on the second, people with actual experience with how parachutes and planes work added interesting techincal details, and on the third page, people were elaborating on why they made the choices mentioned on the first page.
When the first a-hole post a joke response just to bump the thread to get attention. Say, about seven replies. 
It depends on the thread.
Some threads (largely in General Questions) are no longer interesting after the first few responses, while others (often in forums with a lot of debate, such as this one, Cafe Society, or the BBQ Pit) can last for pages and stay of interest.
In general, however, if the thread made it to Page 2 without you in it, it is generally too late to join in.
[Moderator Hat ON]
To IMHO!
[Moderator Hat OFF]
Well, there was the legendary a thread, which started out as an error and made it to a hundred pages before being closed, but that is kind of an exception.
I totally disagree. If I have a fact or observation or question to add to the discussion, I’ll jump in on page two or later.
A thread where a game is going on and only the last couple posts matter can be as long as it wants.
A post where a squence must be read through,I find about a hundred the maximm size.
If I jumped in on the thread early on I’ll usually follow it out until or unless it devolves into something that deviates from the main focus of the OP, such as a discussion of quoted replies amongst only a few posters. If it’s a thread I started myself I’ll follow it through unless, again, the focus is lost.
If the thread goes past two pages I’m not likely to jump in as what I would contribute has likely already been covered by someone else. If the thread itself is longer than four or five pages and I’m late coming to the party I’ll probably not even bother to open it at that point unless it’s a topic that really captures my interest.
Long threads tend to have simplified into two or three determined dopers passionately frothing and boiling. I might open one if the topic is interesting, but will only post on the rare occasion that there is something new to say, and if it looks like it might even get read. Too many people post without having read the thread.
If I see that a thread includes line-by-line quote dissection, creative parsing, or esoteric syntax analysis, I close it quick, shut down the browser, empty the cache, and do a hard reboot.
I prefer shorter threads, and am actually disappointed if a shorter, more focused thread turns into a morass.
Now the real question is why so many threads keel over dead immediately after I post to them.
This is my thought exactly, except my limit is usually once it’s gone beyond 100 posts.
However, what generally happens is that Jakeline, who does not view this the same way I do, will say, “Did you see the ‘xyz’ thread today?” So I still catch the good ones when she directs me to them. 
Ugh. Me too. I rarely read GD because of it.
To me, a thread is too long when the only people still posting are a couple of die-hards with axes to grind. As long as lots of people are still interested enough to post, then the thread isn’t too long.
I like the long threads. I like to see a discussion develop, complete with tangents and hijacks. I get bored when I try to read other boards where the threads are only a dozen posts long.
[hijack]
Holy shit, Soylent Gene is one damn funny name.
[/hijack]
If there’s a third page, I usually don’t bother, unless the thread title seems intriguing to me. If I am already engaged in a discussion, though, I’ll continue.
Occasionally, I’ll end up reading a thread I’d previously skipped because of length if someone in it gets pitted, though. I hate to miss a good trainwreck.