I don't think you need to worry about it much. I sent out my impressions the day I got them done and around 2 months later I tried them once just to see and they fit fine just like the day. I heard it changes a little bit the first day or so and after that it will be like that for a long time.
I'm assuming ear impressions. If left outside, a few days to a week but they can be chilled and will keep far longer according to my audiologist. I had mine sent overseas and didn't have any problems when the product was sent back. I think it took over the weekend and a few days to reach there.