If supply can keep up, before the end of its first fiscal year.

March I'd say.

Really depends from Switch stocks, my best guess would be Q1 2018.

I believe Switch will outsell Wii U in Q1 2018 at the latest. Nintendo's production might be limited right now, but I think they we will see a steady production increase during the rest of the year. My guess is monthly sales for the rest of the year will look like this;

Juni 600 000-800 000
Juli 800 000
Aug 800 000-1 000 000
Sep 1 000 000-1 200 000
Okt 1 200 000-1 500 000
Nov 1 500 000-2 000 000
Dec 1 500 000-2 000 000
Jan 800 000-1 000 000
Feb 800 000-1 000 000
Mar 800 000-1 000 000

My most conservative figures suggest sales will reach 13,8 million units by the end of this fiscal year (March 31 2018), suggesting Switch will outselling Wii U in the second or third week of March.

My most optimistic figures suggest sales will reach 17,3 million units by the end of this fiscal year, suggesting Switch will outsell Wii U in the third week of December.

I'd say the most likely scenario would be for Switch to overtake Wii U sales in February 2018. That's my bold prediction ;)

It feels like it has already...

1st week of February 2018!

December.

It could easily happen this year but there are extreme shortages and Nintendo is unable to produce enough consoles. How many units can they produce per month, currently 600.000? So I voted for Q4 2018 or later.

Maybe around June? It depends on how often they stock the system.

I'd go a bit later than most and say April to June 2018 but its down to supply, pricing, bundles etc. It could be earlier or even later.