@magicheart For this type of patterns I usually use approximately 70 to 80 cm, this gives me a bracelet of 15 to 18 cm. For bracelets with greater detail, more thread will be used, but in this case, the previous measurements can help you. I hope it has been helpful to you. <3 (Sorry for the English, I'm learning.)
@magicheart I usually measure the string from the tip of my fingers and run it across my chest to my other shoulder. I like having more string then I actually need in case something goes wrong (aka my cat chewing the strings lol)