Gonna go with Q3-4 of 2019. X1 sales are slowing overall and Nintendo is set to have a killer 2019. It won't be long now...

A more interesting question would be, will it surpass for the next xbox release =p.

Next Xbox is most likely coming late 2020 so of course Switch will surpass XB1 before next Xbox is released.

The latest hardware numbers (Dec 15th) bring the gap down by 800k to 15.3 million, with the week before christmas still to come.

By the end of 2018 the gap should have shrunk to approximately 14 million.

When Switch launched, Xbox One was at 28.8 million, so that was the initial gap.

28.76m units by 4th March 2017

EDIT: ops, sorry I didn't notice that you edited your post lol

End of 2019 at the latest.

Did the same people who thought Switch would sell like the Wii U also said Never?

It's looking like Switch will have closed the gap by about 8 and a half million units in 2018, a year where it had a huge holiday but spent January-October on cruise control with no big releases.

If Nintendo get Animal Crossing out this Summer, Pokemon Gen 8 in November, and a hardware revision/price cut or both in around October, I think they should be able to sell significantly more Switch hardware this year than in 2018, while Xbox One will likely be down YOY simply due to its age.

Probably not this year if VGC numbers are to be trusted ...

Q3-4 2019

Shipments

GBA　DS　　Switch
FY1　　1.07　5.27　　2.74
FY2　17.09　11.46　15.05
FY3　15.65　23.56　20.00
FY4　17.59　30.31
FY5　15.40　31.18
FY6　  8.83　27.11
FY7　  4.34　17.52
FY8　  1.59　 5.10
FY9　  0.42　 2.35
FY10　0.04　0.13

Average of GBADS　Switch
FY1　3.17　　2.74
FY2　14.27　15.05
FY3　19.60　20.00
FY4　23.95
FY5　23.29
FY6　17.97
FY7　10.93
FY8　 3.34
FY9　 1.38
FY10　0.08