What are the criteria for closing a thread?

They generally close around 35 replies, but it can vary. It's done to keep the bandwidth use at a reasonable level.

It's really not an issue, you can continue the same subject on a new thread (and reference the old one while doing so.)

I don't believe it's the best way to manage it, personally, though I can see advantages. It makes you think "do I *really* need to reply to this?" when you are forced to start a continuation thread, which encourages more new topics. Newer topics with fewer posts encourage more posts (think how many times you can't be bothered adding your twopence worth to a thread with 30+ comments...)

On the other hand, it does seem quite arbitrary, and often means very active interesting threads are cut short, and immediately continued, which kind of makes closing it a little pointless IMO.

Starting part 2/3/4/98 is fine. It is a manual process so it depends when someone is here to close it. A few things we let run like birthday wishes or condolences.