New Super Mario Bros. Wii could (and probably was) done in less than a year
I highly, HIGHLY, doubt it. This is Nintendo we're talking about here.
I have to agree with you, Kairon. From the standpoint of stage design, as well as enemy placement, and comparable things, sure, NSMBWii probably didn't take very long. It's likely, though, that Nintendo put a few people in charge of character design, including those revitalized from prior games, and it's likely the design phases took quite a while. I wouldn't imagine NSMBWii had a very large team until it came time to actually build the stages, and that all the concept work was done by a very small group of people, and it's likely they took a very long time to develop many present concepts.