This is a bit of a vague question, but any approximation would help. When it comes to studying an MBA at a good uni (top 20%, say), I'd like to have an idea of how demanding it can be in terms of time spent studying and working for it outside of class, as I have other projects going on but am considering an MBA.

I know this depends on the specific program, the ability of the student and a million other factors, but any rough idea would help. Should it basically be seen as a full-time job kind of deal?

Cheers!

A full-time MBA is a full-time job. Expect long hours. If you are looking for work, the second half of the course is really taken up with networking, the job search and interviewing more than study. 

Good reply from Duncan, it is a full-time job.  

In my experience, it also depends on your background.  If you did business/management as your undergrad, it will mostly be a repeat of 50-75% of what you learned in undergrad so you can pretty much skate by.  Also depends on how seriously you take coursework.  Some people just do the bare minimum to pass (B is usually the passing grade in US grad programs).   It is very rare that professors fail MBA students unless they were absent throughout most of the semester and did not submit any projects, etc.  

If you are an international student anywhere I say you would have to work 2x harder than your classmates to get similar jobs.  So take that into consideration as well in your schedule.    

If you are asking this question because you want to work part-time while doing your MBA, then my answer is I would not attempt that unless it was something like a teaching assistantship.  TA jobs are flexible and the professors are understanding (will give you time to study).