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?

Actually, I'm not very particular about the sterilizing stays for 6Hrs. So long as it's thoroughly washed, sterilized and kept clean. I will just let my sons used them, so far they are all good. If you are worry, you can use the hot water at the nursing room to go through them before using. Some of my friends didn't even sterilize their bottles since birth, while some stop using sterilizer when their bbs were just few months old. All their kids are also healthy & well now. So, it's kind of subjective i supposed!