Go to Ringcentral.com and then click admin portal and log into your ring central account.
Next you will be presented with this page: 
Then you will want to go to settings in between contacts and more.
Once the settings page pops up you will want to go to user details and then settings and permissions 
Then click edit custom user hours.
From here is where you will set up your hours. Make sure that you have the custom hours option checked. Once you have the hours set up, click save. Now you should only receive calls during those specified times.