The amount of time required to perform Romeo and Juliet varies according to several different factors. Companies make choices that affect the pace of performance in several ways. Pacing is, in fact, an important artistic decision that lends character to an individual production. Several factors affect pacing.
First, not all performance scripts include every line of the play. Some make cuts to eliminate minor characters or stage business. Second, speed of delivery of lines varies, as do the length of pauses between lines. Fight scenes can be drawn out or shortened. Changes of scene can be done quickly or slowly. Actors can respond to others' lines very quickly or allow for expressive pauses.
One general rule of thumb is that Shakespeare's plays take approximately one hour per one thousand lines to perform. At 3,093 lines, Romeo and Juliet would roughly take three hours.
It should be noted that the Prologue describes the play as taking "two hours' traffic of our stage." We do not know, however, if this reflects an accurate measurement of the length of a production of the text we now have. To provide another data point, the run time of a recent Shakespeare Theater Company performance was 2 hours and 40 minutes. The 1968 Franco Zeffirelli film version occupied 2 hours and 18 minutes.