You can set in-app messaging business hours and an away message. When you do, any user who messages you outside of your business hours will automatically receive an away message.
From the left navigation menu, select
[Menu] → Settings.Alternatively, select
[Speech bubble], then 
[Settings].
Under General, select Messaging.
Next to Business Hours, select
[Toggle on]. Then set your availability.Select your time zone, frequency, and business hours.
Type your away message into the Away Message text area.
Select Save.
