My son is 2 1/2 months old and I just started to do tummy time on the floor with him. Did I start too late? I tried when he was even younger, but he hated it so I decided to have tummy time on my chest. He can lift his head and hold it, but when I tried tummy time on the floor I noticed that he doesn't hold his head up like he does on my chest. How long does it typically take a baby to gain the muscle tone they need to lift themselves up?