Common practice for some time with domestic and international air carriers has been to check in for you flights 24 hours before departure, but I am finding more and more that this needs to be earlier, even up to 3 days before. Airlines often make schedule changes–usually subtle ones but sometimes make drastic changes in routing and/or departure times. Relying on them to advise the ticket purchaser is not fail safe. Most airlines still won’t let you print a boarding pass until the day before but avoid being caught off guard with major changes or even cancellations. Time frames vary with airlines, unfortunately–I was just refused checking in online at 23 hours before departure with Lufthansa. It was a very full flight so I risked being bumped.