Agree with Flagg and Jimmy that a healthy adult Corn can happily be left for three weeks without eating, as long as someone changes the water regularly (and closes the door carefully!).
However, younger snakes that need to eat more often - I'm thinking anything on pinkies or fuzzies - or where you have some kind of health concern, probably should be found "boarding accommodation" or have a person with reptile knowledge check in on them and feed them, as well as changing the water.