Your number will be assigned at the 36-hour mark, but you can't see it until 24 hours before your flight. If you go on the site at or after the 24-hour mark and go through the check-in process, it will display your boarding cards and you can check your numbers. You can either print them out then or wait until you get to the airport. Your boarding numbers will be same either way and if you are checking bags, printing the passes in advance generally doesn't save you any time.