We know that the day-one Wii U update is a whopper. It takes quite a bit of time to download. We've also heard that it's around 5 gigs worth of data. Thanks to our friends at Nintendo Gamer, we know that not to be true. Check out this data usage image below, which represents the amount of data transfer roughly halfway through the download process.
Certainly closer to 1 gig than 5, but still a beefy download. Just make sure you set aside the appropriate time to download this necessary update.