The correct answer is "never".
Within the next 3 years, Microsoft will release the next generation XBox. It will be backwards compatible with the XBox One, so technically, I would still consider that part of the X1 family. The next gen release will cause a spike in sales, putting it above Switch numbers.
Second, the poll specifically said Switch, and not Switch family. Within 3 years, Nintendo will release a Switch XL or Switch Lite (some sort of hardware revision). The original Switch model sales will drop, and never pass the sales of the X1 family.
Third, the sales numbers required for this to happen need the Switch to sell Wii-like numbers. I don't see that as being likely at the current price point, but it is possible Nintendo will get more aggressive with price to gain market share.
Lastly, I think that mobile games will continue to erode handheld sales, so the Switch will not follow the standard sales curve. I think it may peak in its 2nd year before slowing down (instead of 3rd or 4th year as is the case with typical consoles).