I'd give yourself at least an hour between the time you expect to be able to get into the security line and your departure time.

At least.

And then start adding time from there. Walk from the daily garage? 10-15 minutes. Check in bags? 20 minutes or so - in case there's a line. It probably won't take that long, but if you rely on a short line and it's not, you're screwed. Remote parking? 30+ minutes.

If you plan on parking in the remote lot, and have bags to check, you'd best plan on getting to IAD by 4 AM or so.