I believe you'd want to wait until the tops begin to yellow and fall over, however, it sounds like they've gone to seed which is not a good thing based on my experience.

Onions are a biennial, which is why those you have in their second year are blooming. Onions are normally harvested at maturity in their first year. I don't know what to do with ones which are going to seed.

Jim

you can still pick and use them--they just will not keep at all, so use them up. The flower stalk ring is huge and wrecks the onions for keeping.