Video – replace Canon 600D with Canon 80D?

From the Canon 80D User Guide, page 310:

Time limit for filming

The maximum recording time of a movie clip is 29 minutes. 59 sec. When the movie recording time reaches 29 minutes. 59 sec. Automatically stops movie recording. You can restart the movie by pressing the button. (The movie is recorded as a new movie file.)

Depending on the film format and frame rate, your movie can now be split into 4 GiB files. This is probably for compatibility reasons (file sizes in FAT32 file systems are limited to 4 GiB), even though the 80D may be expected to use memory cards formatted with ExFAT (which is not limited to 4 GiB file size limitations). The recording time based on format and frame rate is described on page 309 of the manual.