You have a sixty to one ratio on the time sequence (one minute is one hour), so the one week outside should be 60 weeks inside (though you have it backward in the story - one minute inside = one hour outside; but I'll overlook that because I know what you meant).
Would 20 mins. a day for only a month build that kind of muscle? Must have been the potions. But than the Purebloods would have those too, wouldn't they?
The witch is Fleur, right?