What is the minimum distance that allows for shortening prayers (qasr)?

Scholars have determined that whoever travels a permissible journey of four burd (plural of burd), which is equivalent to 83 kilometers, is allowed to shorten prayers. Some scholars, however, are of the view that the matter is not tied to a specific distance, but rather revolves around common practice (urf). So, whatever people consider a journey, it is permissible to shorten prayers and avail oneself of other travel concessions therein, because no explicit text has been reported specifying the distance.