The Core i7-6700K has an unlocked clock multiplier, which means it is possible to overclock it just by changing the multiplier. Besides that, the socket 1151 processors have its base clock independent from the clock of the buses, so it is possible to overclock it also by raising the base clock.
In quick tests, we managed to run the Core i7-6700K with stability (we tested it with Prime95) up to 4.4 GHz, with 100 MHz base clock and 44x multiplier, with all standard voltages. At 4.5 GHz, the CPU was not completely stable.
Anyway, keep in mind that this value was obtained with the standard cooler, not changing any voltage configuration. So, if you use a more powerful cooler and change the voltages (and other advanced options), you may reach a higher overclocking limit.