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 about 160 or so operas - a good deal of the standard repertory plus a collection of assorted rarities.
These running times should be taken with a small pinch of salt. Conductors vary in their tempi and you'll need to factor in intervals/intermissions (both number of and duration) to work out 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...