Since I have a AMD 3900X system, I cannot verify what temps the Intel i7-8700 would run. Basically, temps below 80 degrees are not of a concern. My AMD 3900x actually will peak at ~90 degrees C, this seems pretty common because of the high core count. I would imagine the i7-8700K should be in the 70-80 degree range but I could be wrong.
Another factor to be concerned is the VRM temps. VRMs should not exceed 80 degrees for long periods of time for best practices, but they are supposed to be pretty safe up to 90 degrees. Above 90 degrees indicates a crappy power supply design on the board.