Tip: you can keep a 4Chan thread going longer by bumping it before it expires up to its bump limit. This can result in more people contributing to the thread. Bonus points if you make it less obvious that that's what you were doing.

At the bottom right of the most recent post you'll see four numbers separated by slashes.

For example: 94/11/74/10

In this case, 91 is the total number of posts, 11 is the number of images, 10 is what page it's on (once it runs off of the bottom of page 10 it can be shut down at any time, but sometimes you can save a thread from page 11 if you act quickly.