Around one month to 6 weeks.
Neither of our kids liked to be swaddled and would flail their arms and startle awake without a swaddle, thus, we ended up with two tummy sleepers.
It made us nervous at first, especially as new parents with him, but we didn't have a lot of options besides holding them all night (and I did go through those phases with both, too). When you're desperate for sleep, you will do anything! We did co-sleep, since it makes me feel more peaceful to be able to keep an eye on them and check for breathing!
Henry is almost 13 and still sleeps on his tummy -- er, stomach. He is in his own bed, at least.
Lara
Mama to Henry (6/2000) and Agnes (4/2012)
old school member of the BBB