I'm not comfortable unless I'm at the airport one hour before departure (domestic or int'l from the US, int'l from outside the US 2 hours).
I actually start to get nervous if I'm not there one hour before (even if I'm only one mile from the airport). Especially at airports with off-site rental car returns (and trains, I hate trains). Given the choice of walking or taking a train, I'll walk (much more scenic, and I hate waiting for the train).
Reason
1) I'm crazy

2) I love airports