Level 70-80. You're now in OSRS gold the most difficult part of leveling your fletching. Prior to reaching level 80, you'll need 8323 Yew Longbows. I cannot say how many, but it is quite a bit.
Level 80-85. You have the option of deciding to stick with Yew Longbows and/or switching to Magic Shortbows. Magic Shortbows give you more experience if you buy logs. If you are cutting logs by yourself, it's faster to use Yew longbows over Magic Shortbows. Magic Logs take much longer to cut.
You'll need to create 8,484 Yew Longbows if you choose to stay with them. If you are looking to upgrade to Magic Shortbows, you will have to create 7,620. Congratulations, you did it! Now I'd go take a different route. Perhaps alchemy. or Sell all of the bows you have made to buy RuneScape Mobile gold buy something nice. Or train other skills. Only serious fletchers are allowed to go on beyond this point.