I'm not exactly sure, but it has happened that one of my pet turtles has escaped ans we havent found it for a couple of days. They usually go and hide in dark places like under the cracks of wardrobes or whatever, so we found it and it was all dry and dirty but I just popped it back in the water and fed it and it was fine.
Usually a weekend, but to save your terrapin from becoming very hungry you should get someone to feed him/her at least once a day. I am going away soon and have asked someone to check up on my terrapin and feed it each day. I hope this helped you.