AO810 Card Unused Channels
You can leave the spare AO open as long as you want. There is internal circuit to limt the output voltage when the output is open.
If you short the output, it could also stay as long as you want in this condition.
From the manual - channel supervision only:
Fault detection of the channel. (Only reported on active channels). The error is
reported if the output current is less than the output set value and the output set
value > 1 mA