Is there generally a high amount of variability in the number of hours grad students put to work each week?
I believe that most of the students are event-driven. That is, if they have an important deadline, meeting, exam, TA section (agghh...), then they spend more time to prepare and be ready for that event. Whereas after an important deadline, (or finals week, etc.) they allow themselves to be more relaxed and spend less time "working".

However it is quite strange to define how much time one spends on "research". Many people get very nice ideas just before the go to sleep, or while taking a shower... The mind is running 24/7..  Even though those people are not "in office" they do work on their research.

I agree also with the answer of Lars that it really depends on the student's personality and preferences. I've known students who really need their schedule to be well defined. They showed up to office daily at 9:00 and left at a fixed time (I guess that being married with kids kind of forces you to have a fixed schedule). So there are no fixed rules.