Duplication by SLP is not something you can schedule from the console.
I will propose here two ways to workaround this matter Symantec didn't provide a tool for : one quite well known, and a new one, more elegant but also more discrete.
First Method - scripting
You can deactivate/reactivate SLP duplication operations by scripting the command line :
nbstlutil active -lifecycle <SLP_mySLP> | -backupid <This_backupid>
nbstlutil inactive -lifecycle <SLP_mySLP> | -backupid <This_backupid>
Getting the list of the SLP policies is just a question of nbstlutil list command, while working with the backupid can be more tricky if you want an automized solution.
There you will be able to deactivate all you SLP at night by a cron, a windows task, controleM or $Universe, whatever method is your favorite.
The session_notify and end_session_notify script may help organize the duplication moment too,...