Apple has recently increased the number of days a customer can extend their AppleCare+ coverage to 45 days, up from the previous 30.
According to the AppleCare+ coverage document, first spotted by MacRumors, the coverage can continue 24 or 36 months, or on a monthly or annual basis until canceled.
You can check when your AppleCare+ coverage expires on your iPhone or iPad by heading to Settings > General > About > Coverage. On Mac, click the Apple Menu, then head to System Settings > General > About.