Assume it’s about something at least mildly interesting to you.
Depends what it is. The threads that really appeal to me are things along the lines of “My ____ is broken, what should I do” or “How does _______ work?” Those one’s I usually don’t bother with if they have more then 10 or so replies since I assume an answer was already reached. But I’ll still sometimes stop in and take a look. Other then that, once they get beyond two pages, I often times don’t bother. At that point it starts to get hard to catch up.
Generally, 2 pages (I believe my pages are set at 50 posts) and I have to be really interested to read through it from the start. 3 pages, and it’d have to be titled something like, “Why WhyNot is the most awesome person EVER!” before I’d get interested. 
There have been a few exceptions, although of course I can’t think of any at the moment, where the subject sounds so utterly banal or stupid that I don’t open a thread because I’m so very not interested, and then the very fact that it goes multiple pages intrigues me. Often, these turn out to be very funny threads indeed.
If I’m interested at the beginning and stay interested, I’ll keep up with it into the double digits, but it’s rare in reality for a thread to go into double digit pages.
If I had just responded to your thread title I might have said two or three pages max.
But once the
gets added, then I’m more prone to check it out until I see the thoughts I have on the isuue have been expressed by others or else they prompt me to expand on an idea I have on the topic.
I find I can avoid reading many threads at all based on the title and a mouseover. I’d guess that one out of twenty causes me to open the thread to see a bit more. Once I’m to the point of actually reading the thread I can stay with it as long as it remains fresh or contains things I didn’t know or hadn’t thought about.
A perfect example of a big one that I read all the way through was the one about “not coming to a theatre near you.” That sort of thing – something that makes me laugh at almost every post – gets my attention all the way.
The counterparts are the ones I can dump after two or three posts.
In summary: it depends! 
My vote was for “no limit.”
I’ll skim the first few pages of a topic that’s interesting. Length, however, is less determinative of whether I read a thread than whether I post in one. The longer a thread, the less likely I am to post: a) to post, you should usually read every entry to make sure you’re not repeating someone else, and b) long threads so often devolve into tangents, side arguments, and responses-to-responses-to-responses that it’s sometimes a big investment to get up to speed before adding your own two cents.
But just reading a thread? Read till you get bored then click somewhere else!
“Pages” isn’t a very useful gauge though, because you can set how many posts to view per page in your preferences. Mine is set to show 200 posts per page, for example, so “two pages” means over 200 posts.
I’m actually the opposite way. The longer a thread is the more likely I am to open it, on the assumption that whatever the conversation is will be interesting in some way. I tend not to read the whole thing though, usually just the last few pages.
There’s no set limit for me, since I have my settings set to 200 posts per page. If a thread gets particularly long, I’ll start on the last page and read backwards, instead of starting at the front.
In some ways I am more likely to click on a longer thread, but I usually start a few pages in.
Sometimes it’s curiosity about how a seemingly simple question has suddenly run to 5 pages; other times is because the real ding-dong stuff kicks in after 3-4 pages (i.e. most of the quick replies have been made, so the posters left are the committed / passionate / mental ones which makes for good reading).
Good point. Mine is apparently a default of 50. I’m so used to that that I’ll probably leave it that way. But it’s good to know that “page” is indeterminate.
If it’s interesting, no limit. Right now on my computer at home I have the original Fark thread from when Hurricane Katrina was hitting New Orleans. It’s almost 4000 posts and I’m about halfway through.
Interest trumps size.
I browse fairly often, so if a title is definitely interesting I usually catch it before there are too many replies. Where the number of replies matters is with the not interesting enough to click the first time through titles.
Then it gets to three pages (at 50/page) and I think, there can’t have been that much to say about that, could there? Maybe there was more to the topic than I thought. Should I click now and check it out? Nah.
Five pages - they can’t have been that much to say about that. They’re either starting to argue or they’re starting to joke around and swap puns. It’s tempting. I really like Dopers joking around, but I’m not interested an adding flames to my life atm, and it could have gone either way. Hmmm. . . maybe later. I don’t see how that title could have degenerated into an argument about God or Sarah Palin, but I’ve been burned before.
Seven - All right, all right. I’ll click on the thread. This had better be good.
What? The longer a potentially interesting thread is, the more excited I am before I click. I read too fast, I’m constantly searching for new material.
It depends. In Great Debates, I generally don’t go past 25, since the posts can be longish, and after 25 posts people just keep repeating themselves (generally getting nastier about it). In IMHO and Cafe Society, I don’t really have a limit. I don’t go to the others very often.
Oddly, if a thread would otherwise be interesting to be, I may skip it if it’s too long. If I don’t think I’ll be tempted to weigh in with my own thoughts, I’ll read it if the title catches my eye.
If I haven’t been in a thread since the beginning I won’t even bother reading anything past 2 pages. Once a thread gets that long chances are someone will have already said what I have to say. Plus, I have a really short attention span.
I know what you mean, but the way you asked the question, I could answer one. There are other reasons besides length for not reading a thread.
As for my answer to what you mean: it depends on how much patience I have. But I’ll at least skim up to 3 pages, which for me is 300 comments, so I go higher than you guys do. The exception is IMHO threads, since I sometimes want to express my opinion without getting tainted, Feud threads (duh), and when I’m just not feeling well.
After reading responses:
Also, I find it’s about fifty fifty on whether all of what I want to say has already been said. But I also see no reason not to repeat if I can say it in a different way–assuming some people don’t sem to get it
Considering my experience on the “other” message board? about 2 pages.
(The place was wall-to-wall trolling, flamewars, and personal attacks. Y’all are so nice, polite, educated and sane here!
)