For reference, Wii U sold 13.56 million lifetime, while VGChartz has Switch at just over 4 million as of June 3rd, after 3 months on the market.

When do you think it will happen, and on what do you base your prediction?

It could happen this holiday if Nintendo produce enough stock. But since probably they wouldn't make enough, I'll guess around Feb/Mar 2018.

Between Dec 2017 and February 2018.

At this rate, probably Early 2018

Depends if they could ship another 10 million between now and december 31st.

The question is, will it ever have enough stock to reach that number.

At first they said their plan was to hit 10m first year.
Then they said they would try to increase that number to 14-18m?
But they ran into production issues (getting parts).

It depends on how many more units (than just 10m) they are able to make first year.

I think by Q1 in 2018, they will probably be close or just over.

Probably before the end of the year thanks to Splatoon 2 and Super Mario Odyssey

The problem is they have to fight tooth and nail, to get anything more than 10m done first year.

Because thats all the parts they had contracted, to make units out of.

They said they would try to do 14-18m lateron, but that getting parts was a issue.

They might not be able to do much more than 10m this year.

Since supply is crippling them, I'd say March 2018