This information was supplied by Canon to the largest UK pro-video dealership CVP. It comes directly from the manufacturer’s own testing results.
The Canon EOS R5 does push the technology under the hood very hard. With only passive cooling and no fan, in a body barely any larger than the Sony A7 III, Canon has decided to implement very short maximum recording times for 8K and 4K shooting on the R5. The recording time limits become much more severe once the camera has stopped once on a shoot from overheating.
This is something to be aware of, as it will lead to important shots being missed. Certainly, if you plan to use the EOS R5 for interviews, live events, weddings and wildlife shooting where long continuous takes are required, you may want to stick to 1080p.