this one is a bit difficult to really correlate--it would depend on how much time people are screwing off on the internet and how much time they were doing something productive. I spend quite a bit of time online reading articles and learning about my specialty... which, while its online time, is a positive thing to my GPA (even though i'm actually not taking classes... so that's kind of odd).
As far as my stats:
Online: I figure 30 or 35 hours a week.
GPA: I finished my masters with a 4.0 (in mechanical engineering), from high school through grad school. I'm currently working on my PhD--but we really don't have a GPA because we don't take classes.
Hope this helps!