RUMOR: Nintendo to Halt Wii U Production By End of 2016
Similar to the Dreamcast/Original Xbox's Lifespan
According to Nikkei (translated by Dr. Serkan Toto), Nintendo is set to discontinue Wii U hardware by the end of 2016.
1) The Nikkei just reports Nintendo will stop producing Wii U within 2016. It doesn't say they will necessarily start selling NX in 2016.
2) Acc. to the paper, some Wii U accessories are already not being produced anymore. Nintendo plans to deplete inventory this year. //end
This would be roughly a four-year lifespan for the Wii U. It should be noted that the Nintendo NX will be announced in 2016, though it may not necessarily release this year. Stock of Wii U are still in warehouses and will continue to be made until presumably near the end of the year.
Source: Nikkei Dr. Serkan Toto Twitter