How long should I wait before plugging in a new refrigerator?
The short answer is to check with the manufacturer.  That way you can be sure to stay within warranty guidelines. 

The longer answer is that it varies from model to model.  When a refrigerator is placed in a non-standard position (for example on its side), compressor oil can run out of the compressor and up refrigerant lines.  So if you don't stand it upright and wait, the compressor will pump without sufficient oil -- not good.  

-M