Just completed a roughly 1 hour intervalometer test in 80 degrees ambient, not in sunlight. Using 24-105mm mk1 autofocus off, IBIS on (IBIS on doesn't make sense for most time lapse, but I want WC activity). May repeat with IBIS off.
I got about 800 shots at 5 second intervals, on an old off brand LP-E6 battery before it died. This is quite reasonable battery time I feel, with the viewfinder kept on deliberately for about 50% of the test by leaning cardboard against the proximity sensor, and the screen on constantly for the second half of the test.
I was a bit surprised that the body got pretty warm with 5 seconds between shots- starting to suspect IBIS, screen, EVF or processor itself are main heat generator and am going to try to profile which. No warnings or overheats occurred, but as expected and stated in manual, high frame rate video times were reduced from max capability ( I don't really care, just a data point) due to the body being warmer.
As much as I love the photo capabilities of the R5, it does seem to heat gradually in all modes, and cool slowly. I don't care much about the loss of HQ video modes in most applications, although it is a goofy design. I do care that it can operate in all reasonable environments (to me, that is below 100 degrees ambient with some sunlight). I'm not sure I would feel confident taking this to hot the desert to do a timelapse, where I have run the 5D4 often in 100 degree conditions and up on a timelapse slider. If the R5 gets medium warm after 1 hour of timelapse in 80 degrees no sun, I imagine it might overheat to the point of shutdown even for stills in the desert- I could be wrong but cannot test that now- I may try to get into at least 90s and sun this weekend for at least a 1 hour timelapse.
I usually carry two bodies on long excursions, but would prefer they are both capable of operating so I can shoot several angles. Starting to wish Canon had not been obsessed with the dumb tiny mirrorless form factor (why do people love it so much for pro gear?!) for a camera this capable. Put all this into a 5D4 body and I bet thermals could be much better just for spreading things out and better heat transfer to the body.
I am now repeating the test with my 5D4 for battery and thermal comparison over the next hour. Of course the 5D4 has no IBIS, smaller sensor MP to readout, so it generates less heat anyway, but it is the nearest capable camera I can make a comparison to.