I'd generally say that with much longer threads it's excusable to just read a couple of pages (e.g., first, middle, and last) rather than the whole thing. There are some threads on the forums which are nearing 3,000 pages, and I wouldn't expect anybody to read that much before posting in a thread.
The rule is in place to ensure that users aren't posting in threads with little to no idea of what they're posting about. So long as you're clued up on the thread you're posting in, you should be fine. You're unlikely to be banned for not knowing what the seventeenth post on page 1,408 was.