I just launched a client's Google+ page, and the first thing I did was verify the site url. Just today, only a few days later, I received an invitation to claim the custom url https://www.google.com/+Artbookprinting. With only 3 followers (the +page being so brand new) I would venture to say that site verification is the key component.
Not sure if it is relevant, but I also linked up the company president's google+ profile (also a mgr of the business's +page) with his the site's blog as an author. It's possible that this second level of verification had something to do with being offered the custom url so quickly.
Daniel McClintock
