I'm going for Q2 2018

I'd say Q2 2018, after a little over a year on the market. Really depends on how well Switch holds up this holiday, and if they can compete price-wise with PS4. PS4 has been dominating the last several weeks thanks to third party titles like Injustice 2, etc.. No doubt that CoD, BF2, Destiny 2, and ACO + a potentially $199 PS4 SKU for the holidays, will push some serious numbers.

It's obvious Switch is gonna pass Wii U, and much sooner than later. It's performance this holiday though will definitely be the difference of a few months. 

If they can sort out the stock issue by the holidays then in December it will have out sold the Wii U. If they can't then in February.

I still remember threads from some users that Switch will DOA. :D

Nintendo currently still can't keep up with demand and demand will just be bigger with new system seller games like Splatoon 2 and Mario Odyssey, so holiday season will be very strong espacily with Mario Odyssey. So it really depends of stock situation, without stock problems, at end of this year Switch could easily pass Wii U LTD numbers.

I'd say early 2018

It all depends on supplies. Xmas weeks if Ninty can solve supplies issues, otherwise most probably Q2 2018, as after Xmas Ninty won't be anymore in a hurry, at least for a while, so it won't look for faster and more expensive solutions.