An out of office reply has one reader with one question: should I wait for this person, or should I go and ask someone else? Everything in a good message serves that question, and everything else is decoration.
Three pieces of information do the work:
That you are away
You do not have to say why, and a broad reason such as annual leave is as much detail as anyone needs.The date you are back
Written as an actual date rather than "next week". If you return on the fourth but will not clear your inbox until the fifth, name the fifth.Who to contact instead
With their email. If nobody is covering, say that plainly so the sender stops waiting.
Four sentences is usually the whole message. The most common mistake is not being too short but being too vague, and the second most common is promising to check email when you have no intention of doing so.