It would seem you could harvest several times from one planting using this method -- plants sets, then pull some of them for green onions and replant the root ends, harvest some of those for green onions and replant, etc. The sets that I planted earlier are getting close to scallion size so I plan to give this a try on a small scale.
These onions grew up this spring -- apparently they were so small that once the tops died down I missed them, or just abandoned them. At any rate, i had enough to fill 2 plastic grocery sacks after cutting the tops down quite a bit, sort of humongous green onions. I sliced up about 1/4 of them and fried them to go with a steak dinner a couple of nights ago, saving the root ends for my "project"
As of today, no sprouts yet -- I'll give them another week and then declare my 1st attempt at growing onions from the roots an official failure. They aren't taking up much space in the garden, but I can put it to better use with some radishes or lettuce!