Canon EOS 5DS Instruction Manual page 220

F: Bulb Exposures
Elapsed exposure time
Bulb timer
If you press the shutter button completely while the bulb timer is
operating, the bulb exposure will stop.
Doing any of the following will cancel the bulb timer (reverts to
[Disable]): Set the power switch to <2>, replace the battery, replace
the card, switch to movie shooting, or change the shooting mode to other
than <F>.
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3
Set the desired exposure time.
Select the hour, minute, or second.
Press <0> so <r> is displayed.
Set the desired number, then press
<0> (Returns to <s>).
4
Select [OK].
The set time will be displayed on the
menu screen.
When you exit the menu, <J> will
be displayed on the LCD panel.
5
Take the picture.
Press the shutter button completely,
and the bulb exposure will start and
continue until the set time elapses.
To cancel the timer setting, set
[Disable] in step 2.