Meaning of "nine-to-five kind of person"?

Generally speaking it means someone who works (or would prefer to work) traditional working hours (9 AM to 5 PM, Monday to Friday), probably in an office environment. From this we can often infer several things:

• That they would prefer not to work evenings, early mornings or weekends, at least not on a regular basis.
• That they work for one organisation for an extended period of time, not for themselves or a wide range of clients.
• That they (willingly) work the hours required of them each week, but no more.
• They consider their job as a means to pay the bills rather than a passion. In other words they work to live, not live to work.

Obviously, not all of the above will be true for every individual. Arguably a builder, a shop assistant or a freelancer could be a 'nine-to-five kind of person' if they worked regular weekday hours.

Depending on context, this term can be have a positive meaning (e.g. a loyal and reliable employee) or a negative one (e.g. unadventurous, boring). In many cases it is just a neutral term indicating the person's preferred job type.

Nine -to-five:

• a job with normal daytime hours. I wouldn't want a nine-to-five job. I like the freedom I have as my own boss. I used to work nights, but now I have a nine-to-five job.

often used to mean:

• reflecting, or exhibiting a lack of willingness to work beyond the required amount of time or with more than minimal effort: With your nine-to-five mentality, ...

Source: www.idioms.thefreedictionary.com

OK, I wasn't going to post as an answer, but it seems that the other answers miss the mark.

Summary:
A "9-to-5 kind of person" is someone who prefers to live a structured life. By comparison, someone who does not like "the 9-to-5" prefers a varying schedule and a more unexpected life.

What we have here is two concepts, both with connotations.

9-to-5
Literally, this is referring to a job that has the hours of 9am to 5pm. However, it is rarely (if ever) used literally. The term is used as a way to indicate a type of job that tends to be very structured and rigid. Because of that, it often has the connotation of being boring. When you "go to a 9-to-5" or "put in the 9-to-5", the speaker is usually implying that they do not enjoy their job because of the structure. It's generally considered a derogatory term.

With that said, the job itself is rarely 9am to 5pm. Often times these jobs can stretch to include nights or weekends. Employees who work 60+ hours a week may refer to their jobs as "9-to-5" jobs. However, it's rare for an employee who works varying schedules (20 hours some weeks, 60 in other weeks) to refer to their jobs as "9-to-5" because of the variations.

It's the structure of the job that implies a "9-to-5" type of job.

A 9-to-5 kind of person
This is the type of person who wants the structure provided by a 9-to-5 job. There are various reasons to want that type of job. Often, it's job security, a regular schedule, or a regular paycheck (and often all of those). The reason that "a 9-to-5 kind of person" is not considered derogatory is because working "the 9-to-5" has the advantages mentioned above.

People who are not "9-to-5 kind of people" tend to prefer the varying schedules and desire the unexpected. By comparison, "a 9-to-5 kind of person" tends to prefer the expected and therefore seeks out jobs that maintain that structure.

Personally, I see no negative connotation in the term 9-to-5 at all, especially in this context. The writer is stating how much he loves his job and that his "work" actually involves doing something that everyone else has to pay for. He's just stating that his hours are not normal day-job hours.

If anything, I'd say that "9-to-5" is usually used as a term to describe a boring job - not the poor unfortunate that has to do the job. Take this example from Michael Jackson's "Off the Wall":

 "Tonight, gonna leave that 9-to-5 up on the shelf and just enjoy yourself."