I'm using avent steriliser. it stays up to 6hrs as stated. if I'm going to be out the whole day, how? I can bring out sterilised bottles but I'm staying out more than 8hrs. how?

Agree, don't worry about bottles not being sterilized because once you open the sterilizer, it's no longer sterile anyway. If you're still concerned then just get the bottle rinsed with hot water from nursing rooms or restaurants.