how long should it be?
It depends on how advanced you are at 3a. If you’re just starting out I’d recommend waist-length or shorter. You should feel in control of both yoyos. Once you get better and want to get into more advanced 3a tricks you can make your strings longer. Basically it takes a lot of experimenting, and even for me after many years of playing 3a I still change my string lengths occasionally. I would suggest you buy a cheap 100 bundle of string and cut some different string lengths and see what works for you
^this
I usually cut my strings around pocket height. (the bottom of my pocket to the floor)
Right below the bellybutton for me.