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.
With the Kandarin Medium Diary completed, you spin flax 33% faster at the spinning wheel in Seer's village. A full trip of 28 flax takes 65 seconds round trip. However, full graceful and high agility level is desirable.
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 passing through a stair, thus cutting down clicking times and increasing the efficiency of spinning. In order to use the spinning wheel, players must have started The Fremennik Isles quest and have reached the part where they are able to go to Neitiznot.
Spinning flax into bow strings is a good way for players with low skills to make money. Because of this, bow strings are commonly made by macroers, as they can easily be sold in bulk and for fairly good profits. Therefore, it is quite common to see macros in Seers' Village picking flax in the field, and spinning them into bow strings at the spinning wheel at Seers' Village.
The best way for Ironmen to get bow strings is the Temple Trekking minigame, even with low combat stats. Picking the easiest route with the easiest follower allows you to avoid all combat events but still net up to 3k bow strings an hour.
If you decide to pick and spin the flax yourself, having 30 agility and 0 kg as weight, you can average around 840 bowstrings per hour picking flax at land's end and spinning it at Lumbridge, 740 bowstrings per hour picking flax at Gnome Stronghold and spinning it at Lumbridge and 650 bowstrings per hour picking and spinning flax at Seers' Village.
It is also feasible to pick flax in the Southeast corner of Lletya, spin it at the wheel in Lletya, and bank. This is not common, as access to Lletya requires starting the quest Mourning's End Part I, which has many requirements that include high combat and agility levels and the completion of several other difficult quests. However, this would be a good place to train crafting for players who have high combat and agility, but low crafting.