It doesn’t matter whether you’re going to be out of the office for a few days, or a few months – if you’re going on vacation, it’s critical to remember to set up your out-of-office autoresponder.
Meanwhile, I do sometimes put up long ones when I will be away for some days during a season in which there are a confluence of three or four very likely reasons someone would contact me, and who else they need to contact isn’t the same. Like, it’s high llama grooming season, and generally during this month I get two or three requests per week for each of llama bleaching (for which my backup is Stella), llama shaving (for which it’s Arturo), and llama perming (for which it’s Carter). My message says I’ll be out until blah blah, and if it’s not an urgent llama grooming issue, I’ll get back to you after that, but meanwhile, for urgent llama grooming here’s who to call.

The problem with that is people just don’t look at your signature. Whereas they are reasonably likely to notice the OOO message in the email subject header.
Eh, my team’s instruction to put them up if they’re going to be away from email/voicemail for more than an hour (standard lunch break). I have a ton of staff, and we’re in a business where a high degree of responsiveness, especially during the business day, is expected and few of my staff have mobile email. We’re also a larger organization with mixed project teams, and not everyone knows who’s PT/FT or on nonstandard hours.