How early should you get to the airport in the US?

There is no single answer. You need to allow time for a number of individual things, which may or may not apply to you. Between the moment you drive on to the airport property and the moment you reach the boarding gate, you should allow for:

• parking your car and getting to the airport building. 0 min if you're in a cab or being dropped off, up to 30 min if you need to wait for a shuttle bus and it's a large airport, with the shuttle stopping at all the other terminals first
• checking in and getting a boarding pass. 0 min if you did that at home. 5 min if you're in business class or otherwise qualify for the express line. Up to an hour at busy times for everyone else
• dropping off baggage. 0 min if you're travelling carry on only, or included in your checkin time if you didn't check in online. Up to 20 min at busy times.
• just to make this answer applicable outside the domestic US, there are sometimes immigration formalities like "you are exiting Schengen" in Europe or "pre-clear to the US" in Canada that can involve another lineup of 15-60 min.
• Lining up for security. 5 min if you qualify for the express line. 30 to 60 otherwise at busy times.
• Travelling from security to the boarding gate. 5 min walk in a small airport, 15 or 20 min in a large one with trains and such to get you there.

So best case, I can get out of a cab and be at the gate 10 minutes later. (In a tiny airport, checked in online, status, no luggage.) Someone else with no status, a car to park, a bag to check, and no fast passes into express lines might need hours to cover the same distance if the airport was very busy. At the end of all that time, you need to reach your gate within the time specified by the airline. There may also be deadlines for checkin or baggage drop-off - these should be on the airline web site. For example you might need to get to the gate 30 min before the flight leaves, but drop off your baggage 60 min before. So you should keep those in mind as well even in a small airport.

I usually arrive between 1.75-2hrs before an international flight and I've never missed a flight. Although I wouldn't arrive any later than that (don't need to cause extra stress!). Different airlines can also close their check-in counters at different times so you don't want to be cutting it too close.

Arriving 2 to 3 hours early is unnecessary and neurotic.

1.25 hours is optimal. That is plenty of time to check in, go through security, find the gate and sit down and wait to board.

Eat something before you go to the airport and make sure to use the airport's facilities at least 15 minutes before boarding the plane.

Definitely bring some headphones or earplugs because the loud noise of air rushing over the plane's hull can drive you insane.

It is recommended to sleep on the plane to prevent jet lag if you are traveling East-West or West-East.