How long does a bottle stay sterile outside of the steriliser (for purposes of going out)

[NewMummee]

Usually I don't go out that long so if I'm out I take a ready made milk and a bottle from my steriliser empty but fitted and wrapped in a bag to keep clean. I heard this is ok for about 4 hrs out of steriliser ( not sure if that is correct)
So what If I wanted a day out to seaside or something,where she might need say 2 feeds while out ... What could I do?

assemble the empty bottle and it should be ok for 24 hours. Stick it in a freezer bag if you want to keep the outside clean.

24 hours. The bacteria you are sterilising in order to kill is in the powdered formula not the air.