I vote late 2020. Switch is going to sell 15+ million for the next four years. (Unless something stupid happens)
The gap is like less than 30 million now so it shouldn't take too long.
The Switch will get handheld and console 1st party support. Nintendo is basically throwing their entire weight on this thing. I also expect 3rd party support to grow as time goes on. So the Switch could outsell the Xbox One even sooner.