'before' or 'until' or 'till"

Only the first sounds absolutely right to me.

Do you have any concrete basis or this is this a personal preference?

They are one and the same. An advantage of being a native speaker and English teacher. I don't have to rummage through grammar books all the time.

Personally I would have thought 'until' was fine, and by virtue of that 'till'; 'before' would have been my last choice - if at all. The usage of 'before' (in terms of time) seems to be as such: A before B, where A is an action or nonaction (either continuous or instant) during the period of time prior to B occuring. "It wouldn't be long" does actually state an action or nonaction - which is why

We say "five minutes before nine" and "five minutes till nine".

At 8:55, the time is five minutes before nine, so five minutes will have to pass until it is nine.

Mister MicawberAn advantage of being a native speaker and English teacher. I don't have to rummage through grammar books all the time.

This is not informative. It is merely boastful and dogmatic.

Yep! That's me. And I don't have to ask questions here, either.

I would never write, "Yep! That's me." It makes you sound like a yahoo.

Yep! We yahoos always talk like that, dude.

this one sounds so right...

It wouldn't be long before they started shooting.