In my opinion, the actual amount of time it takes to perform Romeo and Juliet (or any play, for that matter) is variable.  The length depends on a myriad of things:  the speed at which the actors recite their lines, the director's artistic licence for abridgement, the number of props the actors interact with, the amount of scenes containing extraneous events (such as dancing, with no dialogue), even the size of the stage can make a difference.  When one thinks of the time period during which Shakespeare wrote, the lack of props, and the tiny thrust stage on which they were performed, perhaps a shorter time frame seems more appropriate.