I thought I read on here at one time that threads would only be allowed to get a certain length before being closed. But I haven’t seen that being done. I’m not complaining, I’m just curious if I was mistaken.
There doesn’t appear to be a physical limit, beyond the storage capabilities of the server. I’m sure there’s also indexing limitations in MySQL or variable size limitations in the PHP scripts which drive the board, but I can’t state them off the top of my head. The infamous “a” thread went to 107 pages (at 50 posts per page) before it was closed. AFAIK, there is no set limit on what the staff will allow–it likely depends on content, participation, the phase of the Moon, and what have you.
Shall we find out? 
I think there is a limit to the size a thread can get before the mods will close it because of it’s size alone.
Wasn’t the thread about different authors doing lord of the rings much longer than the a thread?
When the boards were running a much earlier version of the boards software, threads generally started acting wonky aroun 10 pages, so they were locked and a new one generally started. This hasn’t been the case in a couple of years or so.
The a thread holds the record for number of posts, but most of them are very short. The Lord of the Rings thread has fewer (though still an impressive number) posts, but many of them are very long, with the result that the Lord of the Rings thread is longest in terms of number of characters or megabytes.
You’re asking: How much thread can a thread-writer write right if a thread-writer would wright thread rightly?
Anyway, what porcupinesaid, the old system had problems with long threads, hence the rule. The new system doesn’t, hence the lack of enforcement.
I do feel, generally speaking, that if it’s a thread with a handful of the same people (like some of the game threads), it doesn’t matter how long it gets. Newcomers, however, probably don’t like reading through 50-page threads before commenting, so the longer threads are more likely to get new posters who haven’t read the whole thread.