A full load of 28 flax will generate 420 Crafting experience (15 x 28).
It takes about 50 seconds to spin a full inventory of flax. It takes approximately one minute and fifteen seconds (1:15) to do a load of flax in Lumbridge. So you can do 48 loads per hour, which means you can do about 20,160 experience per hour, if you stay at a constant rate.
Historically, the only reasonable place for anyone to spin flax into bowstrings was Seers' Village, however with the addition of the bank at the top of Lumbridge Castle, it became on par with, or slightly better than, Seers' Village, but only if you pre-picked the flax and placed it in your bank, otherwise it would be extremely counterproductive because Seers' Village is the most financially efficient place to pick and spin all at once. Later on, an even better spinning wheel was added, this time on Neitiznot. While technically further from a bank than Lumbridge, travelling to the bank in Neitiznot does not require climbing any stairs, thus cutting down clicking times and increasing the efficiency of spinning. In order to use the spinning wheel, players must have started The Fremennik Islesquest and have reached the part where they are able to go to Neitiznot. Currently, it is also possible to pick flax from the field in Taverley and deposit bowstrings at the bank counter at the small gnome tree in Burthorpe, as there is a spinning wheel along the path between the two. Lletya is one of closest places for flax, and a spinning wheel in range of the bank; yet you need to have started Mourning's End Part I to access it.
Spinning flax into bowstrings is a good way for players with low skills to make money. Because of this, bowstrings were commonly made by macroers, as they can easily be sold in bulk and for fairly good profits. Therefore, it used to be common to see macros in Seers' Village picking flax in the field, and spinning them into bowstrings at the spinning wheel at Seers' Village.
The item was originally called "bow string", although "bowstring" was also used in the examine text of unstrung shieldbows. In November 2013, the item as well as all references were changed to "bowstring" for consistency.