Some operas are very long, and some are very short - the shortest we know of, L'Enlèvement d'Europe, is a mere 8 minutes! Our list currently has over 200 operas - a good deal of the standard repertory plus a collection of assorted rarities.
You should take these running times with a small pinch of salt. Music can be cut, Conductors vary in their tempi, and you'll need to factor in intervals/intermissions (both number of and duration) to determine how long you'll be at the theatre. In most cases, we give the running times on an Act by Act basis except in cases where the Acts are conventionally run together (anyone who sticks a break between Acts III and IV of Aida should be shot). We've also tended towards longer running times rather than shorter: leaving the theatre earlier than expected is pleasant whilst leaving late means missing the train...