Good question. I think it has a lot to do with your relative humidity. The ability of onions to keep in storage also is dependent to some extent on your relative humidity. Here with our relative humidity around 20% much of the time, a week of drying the onions is enough. They will also keep all winter hung in an onion bag down the basement.I've got mine in a single layer on 10/20 flats, but I'm not sure how long the let them "air out" for. 2 weeks maybe?
If the tops seem very dry and papery and the roots are brittle, you are dry enough.