About half an inch to an inch is sufficient - you don't want them much longer than that because its harder to pot them without damage, and once roots start appearing, its surprising how fast they grow. Try not to damage them when transferring to a soil medium - make a hole large enough to accommodate the rootlets without squashing or bending them, and gently push the soil around them, pressing down lightly at the base of the stem when you're done. Water in well with the pot standing in something to catch the excess - empty that after 30 minutes. Best done into a potting medium that's already damp.
Now you've added pics, wait a bit longer till they're half an inch, although, as its Epipremnum (Pothos) it will be okay to pot it now, but it means you have to pay particular attention to watering - enough to encourage the roots, but not so much that a fungal infection occurs. If you can bear to wait, it shouldn't take long for them to grow.