How long can onion seedlings stay in their cells?

[SLC]

I have one of those 72-cell self-watering seed starting trays, and the cells are pretty small, maybe 1" wide by 3" deep. And I thought this would be perfect to start onions. Is it?

My question is, how long can the seedlings stay in there before they have to be transplanted?

I guess the important question is when does the bulb start form? Cuz there won't be any room in this tray.

[jal_ut]

I would plant them out when the stem is about the size of a stick match. They won't bulb until the days are of the right length. June probably. They will be pretty good size by then.