Domestic ~45-60mins
International >120mins
Varies depending on what I'd planned earlier that day and whether there's anything I need to do at the airport (e.g., I often get thirsty...
I've made a flight (with checked luggage) with 3 minutes to go, but it's not recommended...

(Even the luggage made it too...)
On the other hand, I've made it there as much as 4 hours prior, because I'd nothing better to do.