puppy food onto adult

Hi everyone have been feeding sasha twice a day and she has 0ractically the maximum amount for the age on the back of the packet but a coyple of hours later she is still hungry so we give her a banana or apple. Took her for a run over the park today and she spent most of her time sniffing around for food on the floor she wasnt interested in her ball at all. Could it be possible that the puppy food just isn't filling her up and what age would be ok to change her onto adult food I know the packet says 12 months but am curious to hear your views on the subject. She has wainwrites puppy with a small amount of wainwrites tray mixed in twice a day and she is now 7 1/2 months old. Your advice is as always greatly appreciated xx