It is up to the user (the server can't enforce or see) but clients will use a default value.
For a long lifetime coin, this should be months.
For the DIFFERENCE on transfer:
It needs to be long enough to get the backup confirmed in that time. Assuming a CPFP to bump fees to the required fast confirmation level, this can be a short as a few hours.
However this becomes less secure for low value coins (where fees might be high in the future).
It is up to the user (the server can't enforce or see) but clients will use a default value.
For a long lifetime coin, this should be months.
For the DIFFERENCE on transfer:
It needs to be long enough to get the backup confirmed in that time. Assuming a CPFP to bump fees to the required fast confirmation level, this can be a short as a few hours.
However this becomes less secure for low value coins (where fees might be high in the future).