Usually I use as much of the green onions as possible, but if you only want to use the green parts then consider saving the bottoms (the white part) to keep growing your own.
Start by leaving about an inch or so of green parts on each onion,
Place in a glass with some water and leave in a sunny spot.
Within about 12-14 days you'll have a whole new bunch of green onions to use!
I have regrown mine several times and I honestly don't know how long you can keep growing them on one stock. If you do this too, how long does yours last?