I expect Gods and Kings to get 6 - 12 months active support from Firaxis, at the end of which time development of Civ 6 will be well underway, and I predict we will see Civ6 by 2014 (the year Windows XP SP3 dies).
God knows what Civ6 will have in store, I actually think Civ5 is the pinnacle of civ games so far.