It should take about 4 years if sales stay the same, but average PS3 sales are on the rise and average X360 sales on falling so it will most likely happen next year and probably at the holiday times when the most consoles are sold.
End of 2014 Hardware Predictions (03/03/14)
PlayStation 4: 12-15million
Xbox One: 7-10 million
Wii U: 8-9 million (Changed 01/04/2014 from 7-9 --> 8-9 million)