I would give it 24 hrs.
You want to avoid thermal shock from sudden temperature changes. And a sudden change to the humidity.
It's not so much the temp from its point of origin to your destination, it's all of the temperature extremes it likely has gone through on its travels. (Though if you live in places that are really cold or really hot, that can mean more time in extreme temperatures and humidity.) I've received two ukes in the dead of winter with low temps of -28 degs F. They spent the night in an unheated warehouse shipping container. And at least one day in the delivery truck. Often the ukes are traveling cross-country in an unheated semi truck trailer.
The box, packing materials, and the instrument case all act as insulation. Slowing the changes to more gradual ones. If you receive and open the box and case immediately the sudden changes in temp & RH can do all kinds of damage from splitting the wood in the top or back, to warping the neck/fretboard.
If I know the instrument is in a case, I will generally open the outer box after 4 to 6 hours. To make sure the case is undamaged. That is usually enough time for the outside of the case to get close to room temp. If there is a humidity barrier bag, I will remove that. I then put the case back in the open box and leave it for the remaining 18 hrs or so before I open the case. I find that gives everything enough time to equalize and acclimate slowly.
I then open the case and inspect the uke to make sure it is undamaged. I'll often place the tenor back in the case and wait until I have enough time to tune it and play it for a while.
That's my MO. Some people give it more time. Some less.
I usually photograph the box when I receive it. And then each step as I open it. I also photograph the instrument as I inspect it. That also helps if there is a damage claim with the shipper.
If the box is damaged when I get it, I open it to make sure the case is intact. (The shipping companies can sometimes get a little testy if you wait to report a damaged item.)
If I'm buying a used instrument, I let the seller know when I get it, and what condition the box is in.
And then, the next day after I open and inspect everything, I let them know if all is good or not. If not, I send some photos to them showing the problems along with an explanation of what they depict.
I've had issues with damage from shipping three times out of 20+. In each instance, it has been a bizarre circumstance that has caused damage to the instrument.
The weirdest involved the key to the instrument case getting out of the middle compartment and moving around inside the case. Scratching and scraping various parts of the tenor! The key had very sharp flashing around the edge on one side of the key. So, in the course of it's travels in the case it shaved a small area of the finish on the back of the neck, and nicked the clear finish in several spots on the body. Mostly on the bottom of the uke. I found the key sitting in the bottom of the case after I let the seller know what I had found. He called me and was absolutely mortified and floored that the forgotten key could do that much damage!
Stay tough and wait it out. You'll be happy you did.
