There is also a link called Manage, which I didn't notice by now, so I clicked it. That action opened a very useful page, called Drive storage. On this page it is indicated "Total storage", and possible extension plans for your account. The plans are starting from 100GB for $4.99/month, and ending with 16TB (the rough equivalent of 1,000 free accounts) for $799.99/month.
The more important thing to consider for a free account is mentioned on the lower part of the page.
- Google Drive: Store files up to 10GB each. Anything you create with Docs, Sheets, or Slides won’t use up any of your storage.
- Gmail: Attachments sent and received in Gmail as well as your email messages use your storage.
- Google+ Photos: Photos bigger than 2048x2048 pixels use your storage. Everything smaller than that is free.
UPDATE: There's a new Gmail inbox starting today (June, the 20th, 2013):
- organized into categories,
- easy to customize,
- available on your mobile device.