Depends where from, which airline, pre-check or not (in the US), and whether you have luggage...
If you're checking luggage, does your airline have self-service (i.e. you tag) or semi-self-service (you push buttons, the employee tags the bag) kiosks to tag bags, or do you have to wait in line behind the family of 7 with 38 bags on a 4-segment flight to Nowhereland who couldn't be bothered to pick seats online and is now crying to one of the two airline employees on duty that they can't sit together?
How familiar are you with the congestion at security? If it's your home airport and you fly at the same time of week often, you've got an idea how long (or not) security takes.
etc etc.
I always check in online and rarely check luggage, so I've been known to turn up 45 minutes before the flight if I'm reasonably certain I won't get stuck in a security or outbound immigration control line. That said, in an unfamiliar city where I'm taking transit I have no experience with, I might show up 2.5 hours early and hang out in the lounge and/or bar rather than running the risk of missing the flight.