Canon EOS-1D X Mark III Advanced User's Manual page 282

Movie Recording Time Limit
When recording non-High Frame Rate movies
The maximum recording time per movie is 29 min. 59 sec. Once 29 min. 59 sec. is
reached, recording automatically stops. You can start recording a movie again by
pressing the <
When recording High Frame Rate movies
The maximum recording time per movie is 7 min. 29 sec. Once 7 min. 29 sec. is
reached, recording automatically stops. You can start recording a High Frame Rate
movie again by pressing the <
> button (which records the movie as a new file).
> button (which records the movie as a new file).
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