Global setting to change rings before going to voicemail for all mailboxes

As far as I know, no global default setting for this sort of thing. Only way to sort of do this is by using the templates feature (Company Info/Account Settings/Templates). 

Make a User template that sets the max ringing time to the time you want, then apply the template to the user extensions and in the future, use the template when creating new user extensions. 

Note that applying a template to an existing user extension will change the other settings specified in the template as well; no way to update only the ringing duration.

Another method would probably to write a program to use the API; but obviously that is a bigger project...