My son is 3 weeks and started doing it on his own at 2 weeks
Without it flopping after a few seconds or her going flimsy, at around 2 months. Started holding herself up duting tummy time 2 months 1wk, rolled freely front to back and back to front completely at will by 2 months 3wks. She is 3 1/2 months and spins on her belly to change directions and inch worm herself toward a toy out of reach.
My daughter sleeps on her tummy. She started holding her head up at 4 weeks.