Currently there are no user hours settings for any notifications in RingCentral. Hours can be set for your incoming phone calls but it you wish to be notified of a missed call or a fax, the notification (email or SMS) will come through as well.
While it might be a nice feature to be able to set hours for any notifications (SMS, phone call, fax or missed calls), there are numerous apps which can handle that.
I don't have an iPhone
but Android phones have numerous options. One of the best, which I use,
is AutomateIt Pro. There is a free version but the paid AutomateIt Pro
(which is only $3.99) enables other features including silencing the
phone during meetings, which was important for me. The free version will silence your phone, based on rules you set up, for any time periods.