The best time to reread a difficult passage in a text often depends on your specific learning style and the context, but generally, a good approach is:
After reading all of the remaining text.
This allows you to get a better overall understanding of the context and how that difficult passage fits within the larger narrative or argument of the text. You may find that having more information helps clarify the difficult passage when you go back to it.
However, if you feel particularly confused or if the passage is crucial to understanding the subsequent text, it might also be helpful to reread it immediately after encountering it.