I thought it was 12 hours max but I'm still researching getting ready to start my own

I never start onions indoor. But in the nature onions (of all kinds) grow in shorter cooler days, less than 10 hrs daylight(late winter). 16 hours sound way too much.

Isn't it more related to "intensity-hours" versus straight hours of light?

In other words: 10 hours of direct sun is very different from 10 hours under fluorescents.

I really don't know, guys. I have 40W of T-8 lights per sq. ft some 2"-3" above the seedlings, and am worried they'll start getting leggy if I reduce the timer much below the current 16 hrs. On the other hand, I don't want to mess up whatever internal clock they may have.
Surely there's someone here who's grown allium seedlings under lights...

I grow mine from seed for a few months under lights, and then move them to windows w/o lights as all the other seedlings we must get going crowd them out!
I have used light from 12-17 hours a day, and really didn't see much difference in how they bulbed later on. It just works for us, w/o much worrying about it. Sometimes the lights have been on continuously for days! That worked also, it was apparent at harvest time :)
Maybe we are just lucky.