score:3
C and Z are unfortunately on opposite sides of Terminal 1, so it is quite a bit of a hike. Exact walking time depends on gate location (Z is very long) and your walking speed. I'd say 15-20 minutes.
Your best shot is probably to take the airside section of the SkyLine https://www.frankfurt-airport.com/en/travel/transfer.detail.suffix.html/article/travel/terminal-transfer/terminal-transfer-via-skyline.html I'm not sure that there is an airside walking connection anyway.