Thought I'd share this here because I know there are people who appreciate debunking old wives' tales.
Popular thought on a lot of the forums when receiving a guitar by mail is that you need to wait 24 hours before pulling it out of the box. To open it up earlier risks damaging a new and expensive guitar.
I knew 24 hours was far too long and guessed that 8 hours would do it. It turns out even that is excessively conservative. I did a test on a frigid night recently, and found that 3 hours or so is plenty for a guitar to get warm enough to pull out of the box without risking damage.
To do my test, I put a bluetooth-enabled thermometer in a hardshell case with an acoustic guitar, and boxed that up with as many packing peanuts as I could fit. Those peanuts acted as insulation to slow the warming process a bit, and were probably more packing material than is usually used. I then set the box outside overnight to get it as cold as it could get. Minimum temps overnight got down to about 13°F.
Before the sun came up (and things started to warm up), I brought the guitar inside and let it sit.
In about 3 hours, the temp inside the case was up to about 50°, and that is plenty warm enough to avoid any damage. It took about 9 1/2 hours for it to come up to room temperature.
FYI, I attached a graph showing the temp over time.
Hi Eric - that is a nice piece of work, with real data. I like it. Over on the UMGF there have been some threads about this question and all of the answers are the usual mix of speculation and anecdote. You have actually answered the question.
Hi, I would think the reverse is the case in my neck of the woods. I have a calling-down procedure to keep in mind. Guitars are often delivered to me out of the hot back of a courier's van when the outside daytime temperature is 40deg C.