You can print a United Airlines boarding pass up to 24 hours before the departure time, after checking in online. Some timing is different depending on the flight and airport specifications.
Online check-in and boarding pass printing is available for domestic and international flights. There are some requirements for checking in online, including having the first flight in the itinerary being operated by United Airlines. There must also be four or fewer flight segments in the itinerary and fewer than nine people traveling in the party. Online check-in is not available for special requests that require a paper ticket at the airport.