Probably at least a hexabyte. Think of all the songs you know, all the people you remember, all the experiences you remember, all the skills you have learned, plus all of your perceptions that become embedded on you brain(taste, touch, smell, sight, hearing). That's a ton of information. I would even say a few hexabytes now that I think about it.
The term probably does not relate to the storage of information in the brain, since the terms byte, kilobyte, megabyte,gigabyte, terabyte, etc., refer to the manner in which information is stored in electronic digital computers. The brain stores and processes information through a combination of electrical and chemical processes and is much more dynamic and transformational than an electronic digital computer.