It's completely up to you. If baby starts trying to guzzle milk and starts to collapse the teat, then it's time to move up. My friends baby (well, she's 21 months old now!!) never moved up from newborn teats, she hated the faster flow. My LO moved from newborn teats to the next size up at around 6 ish months, even then it took sometime "weaning" her on to them as sometimes she would choke herself a bit! But she's fine now as she's got used to them, but I prob won't move up to size 3, she seems perfectly happy on size 2!!