A week had passed in the mansion, and Harry had been training with Merlin excessively. Merlin had cast a time spell on the whole mansion, so for every hour that passed outside it, only one minute had passed inside it. So basically, the last week was like a month for Harry, so he was able to learn a lot.
That just doesn't make sense.
1 minute in the mansion = 1 hour outside
Harry is in the mansion, training.
month in the mansion = 60 months outside = 5 years for the rest of the world
Five years does not equel 1 week.