I have a 6 mo old and switched her to an all life stage food at 4 mo. If I understand correctly the puppy formulas have more calcium that they need as growing pups but if fed too long could cause a condition called knuckling over. I think that happens though when they eat it for a real long time though.
As far as how much...I was told not to restrict food when they are really young. I offered a cup of food at each feeding and picked up what she did not eat. I fed 3x daily until she was almost 6 mo. At first my pup would eat what she needed and walk away. Sometimes she'd eat a whole cup and at others only a quarter. But then she became a little pig and would eat every morsel so I cut back to 3/4 cup at each feeding. At 6 mo I have her eating a tad over a cup in the a.m. and another cup in the pm. I also give her 2 home baked treats daily.