Leaving a 5 month old pup alone for 12 hours is far too long.

See if you can get a dog walking service to let puppy out even once during the day while you are away.
This is such an important time with house training and socialization...and to be left alone for 12 hours a day for 3 weeks is cruel.
What about your breeder? Could you leave puppy at your breeder's for this time?
Gina