This depends on whether you are travelling on a domestic or international flight. As a general rule, for domestic travel, you should check in one hour prior to departure, and if you are travelling internationally, you should check in at least two hours prior to departure. If you are travelling internationally, but have an initial domestic flight connecting with your international flight, you should check in 90 minutes prior to departure. You should always be aware of traffic conditions between your home or hotel and the airport. It is a good idea to consult the airport website to see what they advise for their particular airport, as some airports have more stringent security checks that take longer to get through.