Sterilising is important during the first months because milk germs can linger after cleaning babies' bottles. These bottles can incubate bacteria and cause a lot of infections. Once sterilised, these germs will not return unless they come into contact with the milk.
How long can the bottles be kept sterile ?
If you sterilise your bottle in the morning, they will be ready to use throughout the day, provided you leave them in the sterilizer and keep it hermetically closed. Otherwise put them in a spotlessly hygienic refrigerator.