I don't think there's a limit of pages in this forum. In other forums, a certain limit is hit and people are forced to create a "part 2" of the thread and they continue there.
In here there are many of these multi-page threads, many are filled with people posting nonsense to get some satoshis on signature campaigns. BUT it would be unfair to call all multi-page threads a spamfest. For instance, the Martin Armstrong discussion thread has 200+ pages and there are pretty high quality stuff to read in there:
https://bitcointalk.org/index.php?topic=1082909.0Just ignore low quality threads and you should be ok.
If I understand the question correctly he's not asking whether there's a limit to how many pages the forum can have, but when its justified to create a new topic of an existing discussion.
I don't think there's any set rules on this, but as a general rule of thumb you should only create a new topic of an existing discussion if one of these are true:
- The discussion is off topic
- The thread is self moderated, and you would like to participate in a discussion without moderation from the OP
- The discussion isn't up to date
- You've tried to participate in the discussion by adding new quality discussion but, have received no replies. You may open a topic about this recent development, rather than trying to replace the existing thread.
If it's a question like "Why is Bitcoin better than gold" then this is likely to be always relevant and I don't think it's beneficial to open another topic about it just because there's a lot of pages in the existing one.