It will depend on the variety on how fast they grow. Some are easier to keep in pots and some want to grow up to be big monsters very quickly. If you know the parentage of your seeds then it will be helpful. Seedlings will usually grow for a year or so before they flower especially in a cooler climate. Seedlings can sometimes get pretty tall before they flower as well but not always.

If you get a cutting that has a Y on it or is taken above a Y shaped branch of a brugmansia plant, you can usually get it to flower on a shorter plant. They will also flower sooner. Hope that helps.
~SJN

I am pretty certain the seed packets read "brugmansia suavolens" on them.

Thanks for your response.
Other responses welcomed, too.

Those should be very fragrant. I wouldn't pot up any small plant into a huge container because it might rot. Just pot it up to the next sized pot, and it will eventually out grow it and repot again until it's ready for the big one. I've lost many brugs to a huge pot and rotten roots! best wishes!

They do grow quickly but really need to be up potted like Kathy said at first until they get bigger. I have seen some in the ground grow to 6 feet in a season and others stayed shorter so with seedlings you just have to see how they develop.

I have a white that I planted (seed) in last part of Nov and now it is three feet tall and is blooming the second flower and has more.