The usually say it is best to leave overnight to let the gas settle.
However if you ask any kitchen installer they will tell you that they plug them in almost straight away without any problems.
Providing that the unit has been sitting upright and only moved in that position (ok, a little tilt doesn't hurt either), you should be able to plug it in right away. They did with my new fridge when it was delivered and we had ice made in less than two hours.
Where you run into trouble is if the unit has been laid flat either on it's side or back and left there for a bit. Then you need to let it sit and let "thing" settle back in the right places before you plug it in.
24 hours. You have to assume that at some point in the delivery from factory to shop and then to your house, it might have been on it's side or even up side down. The refrigerent must have time to settle into the compressor before switching on.
I was told 24 hours
It should be left for 24 hours after delivery to allow the gas to re-settle.
12-16 hours at least...