800 hours
Most played game: MK8 More than 200 hours

Just under 300 hours, I have had it since launch, it will go up with Zelda next year

I'll check later today, unless when it becomes too cumbersome, the WiiU Activity Log sucks balls compared to the 3DS one where you can sort and rank stuff and see totals, averages and graphs. It's hilareous, but anyway.

I doubt I'll be over 1000 hours, maybe 500-600 or something. And I have the system since launch day. From the top of my head only Super Mario Maker goes over 100 hours for me, but like I said, I'll probably check later today, because now I'm curious.

I'd guess around 500 hours from Wii U games, with my most played games being Xenoblade (120), Smash (90) and Kart (80). I'm about 5 hours into Tokyo Mirage Sessions, will be doing TP:HD again and have a few unplayed eShop and Virtual Console titles to get through, so I guess we could at least another 100, more if I get Zelda on Wii U instead of NX. I've had the system two and a half years.

Unfortunately my academic work takes up more and more of my life, so with my gaming time split between 3DS, Wii U, Xbox One and PC individual systems sometimes don't get much love. If NX is a portable that hooks up to the TV and has a unified library, that might actually be really good for me. My 3DS is at 1040 hours over five years, overlapping playtime with Xbox 360, Wii, Wii U, Xbox One and PC, plus my retro systems (Saturn, N64, Dreamcast in that time).

how do you find out the amount of hours you've played your Wii U?

For sure! Love the 3DS logbook, Wii U is not up to par compared to it. Still decent though, It bothers me exceptionally that there is none on my PS4.

I did a quick look and know it's over 1500 hours since launch, but I didn't do a perfect breakdown.

Over 1100 hours of just Wii U Retail and eShop games. Add in Wii games and probably closer to 1500 hours since launch.

Top 5 most-played Wii U games are:
1. Splatoon-314 hours (and still going, determined to get to level 50, and that's without playing Ranked cuz I don't like it, currently level 46, I'm good but it's slow-going sometimes when your team sucks)
2. Xenoblade Chronicles X-142 hours (literally all in the span of 4 weeks haha, I don't think I've ever played a game that much so quickly before, but I wanted to beat it during winter break before school started up again)
3. Call of Duty: Black Ops II-96 hours
4. Super Smash Bros.-68 hours
5. Mario Kart 8-55 hours

Just did a quick recount and have a 1250 hours total of usage of the WiiU OS side of things.
Won't bother dividing the hours for gaming, internet and netflix (even eShop/Miiverse) since it doesn't collect Wii mode data.