I leave my apartment in Palermo about 3 hours before my flight. The timing will also depend on:
Weather - if it's raining, traffic on the autopista will be slower with more chances of accidents;
Class of service/status will determine what check-in line you can use;
Whether you want to board at the first opportunity to ensure overhead storage space (if you are in Y).

I use a remise service which always is on time (or a little early) since they're situated a few blocks from my apartment. I have 1K status with UA, so security/check-in only takes 5 or so minutes. On Friday after clearing immigration I had 30 minutes to kill at the Red Carpet Club before boarding commenced.

The biggest hold-up is the security screening, and how many WTMDs are in use. Also with the dozen or so flights departing in the evening bank, there will be crew members cutting in line, not to mention elderly passengers in wheelchairs, and often passengers with children or pregnant women who will be given priority.

John