How long does it take to make a good steady sound on flute?

I just recently purchased a Harmony brand flute. I have been playing for two days, and can get a good sound on the head joint, but with the whole flute together, the sound is unsteady and wobbly, and goes back to what I call a 'wind tunnel'. It won't stay on middle F, and keeps going in between middle and low F, and 'wind tunnel'. I am blowing a lot of air, so much so that I feel dizzy, but it still won't come out steadily, and when I stop the air (tonguing) the sound stops coming out. I know my embouchure is correct, so why does this happen? And is it me or the flute? and if it's me, how long does it usually take to correct this?