usually check in time is written on the ticket, check it. to be safe, be half an hour earlier than the check-in time (or more if you have weak sense of time), that means leave the house an hour earlier or more depending on how far you live. if you're taking a ride though, you should probably leave before 12am since they don't work after that. anyways, bole airport is nice and cozy with free wifi in every corner, being there before midnight won't be sad (unless you're leaving loved ones behind, ofcourse).