Ignoring the rides in Mexico and Norway for the moment, I would say you need a good 20-30 minutes to really appreciate each country, all the shops and interesting spots. Don't forget the movies in Canada, France, and China, and the theater show in America.
If you want to see each attraction/movie, and walk through all of the shops and exhibits, you'll need at least at least a day and a half (a "day" being WS's hours of 11am-9pm) to really do it right. Also, that doesn't include dining - you're really missing out if you don't dine a bit too.
To answer your more specific question, if I had a few hours in one evening, and a whole day, I would probably start touring in Mexico. If it's not terribly busy, you should be able to see everything in Mexico, China, and Norway in one evening (though you might be rushing through the huge gift shop in China, and wouldn't have time for a sit-down meal). The next day, I'd start at the other end of WS (Canada) and work around. Even on fairly busy days, the only attraction in WS that really has any appreciable line is Maelstrom in Norway.
By doing things in that order, i.e. working counterclockwise from Canada to Germany, your last two stops (Germany and Italy) don't have any rides or shows, just food and shopping. So if you're running short on time on your second day, you're missing less. That assumes, of course, that you don't have a special interest in one or more of the countries. We personally love to spend time in Japan and China for the landscaping, architecture, shopping, etc., so it really takes us a long time to get through WS. In fact, with as many trips as we've taken, we still haven't been inside every shop and seen every little nook and cranny, so don't expect to see it all.