You MUST check in at least 60 minutes before departure for an international flight. Otherwise you will be turned away. They have to do the no fly list thing. Otherwise you must check in 30 minutes before departure or you "may" be turned away. In reality, unless they are oversold and need your seat you will most likely be fine. BTDT
I fly A LOT... and sitting around airports more than necessary is not my idea of fun. I normally fly out of a medium size airport and arrive no more than 60 minutes beforehand if flying domestic and 90 minutes beforehand flying international. I never check luggage (even if going on a month long trip) and can be at the gate within 15 minutes of parking in almost all cases. It helps that I have elite status and get thru security a bit quicker than usual but still think that I could get thru in less than 30 minutes in almost all instances at my local airport. Other airports such as MIA I would allow an hour or more.
When returning, ask at the departure airport how early you should be there. Ask several people and average it out, LOL. Some places will take much longer than others.