Someone that commits themselves to a relationship for a short while, at the most a few months and then immediately end it once they feel things have run their course. They move from the end of one relationship to the beginning of a new relationship quickly.
Tom: "Happy 3 Month Anniversary baby?"
Ella: "I love you, but its a new season, so we're through. Nothing personal hun, but I'm usually onto someone new by now. My 3 months are up, buh bye!"
Tom: "Damn quarterly monogamist!"