Whenever I get an email from ao3 saying "_____ left you Kudos!" or "_____ commented on ______" An extra week is added onto my lifespan. Remember to always give ao3 authors you enjoy kudos and comments to ensure they live forever.
I personally always leave kudos after reading the entire fic. But I noticed a lot of people already leaving kudos a few chapters into a story or even in a 1st chapter of a new fic.
Can I just say that kudos-ing a fic on ao3 is so adorable because like it sends an email to the author and like imagine loving a story so much that you send some a lil piece of paper that says "hello, I really enjoyed your story" like Hi! Author-person! I love you and your funky little story!