An unstrung shortbow is cut from Logs at level 5 Fletching, granting 5 Fletching experience. An unstrung shortbow can be strung by using a bowstring, granting an additional 5 Fletching experience for a total of 10 for a single bow. The unstrung bow then becomes a Shortbow.
It takes 15 seconds to string an inventory of 14 shortbows. With banking time, this means players can potentially string 3 inventories a minute, which is 180 inventories an hour, for 2,520 bows in total. This means that the experience gained per hour is roughly 12,600 Fletching experience.