Is it similar to a domestic flight in the U.S., or more like an international flight?
Similar to domestic. Around a hour to a hour half should work.
To disagree somewhat, most airlines recommend two hours for international flights within Europe. It gives an extra cushion if there are delays checking in or at security. (Certain budget airlines are notorious for only having one or two check-in desks open at certain airports, even at busy times.)
90 minutes is enough in most airports. There are no border control formalities.
You didn't mention the airport. It really depends on the airport. For example, I always plan on 2 hours at LHR. Also, depending on your route, you may have border controls (e.g., flying from a Schengen country to a non-Schengen country.)