I think its 4 years on the top of the line models.
I agree with you, though its not that drastic. I got the 1DX II and it feels a bit "dated" in some of its features. Another codec option and a better touch integration (cant use touch in menu, which is silly) would be good. Also intervallometer and 4k HDMI output (only FHD). Waiting one more year for the next model feels a bit long for me.
I am in the silly situation where a C200 would be a bit overkill (I do photo AND video work, so I would love a good hybrid) but the 1DX II is a bit lacking the video front. And sadly the 5D IV, while great in photo, is quite a disaster in video mode. I think a 5D V right now would be great. A bit more resolution, a bit better video features and it would be great improvement. But waiting nearly 2 more years? brr