Is it all assignments from the teachers, or is she also studying and trying to do extra? And is she using her time effectively, or is she mixing in homework with playing on the computer, texting friends, etc.? Is she just being a perfectionist who wants to get straight A's all the time?
Most schools have it set up so that you can view daily homework assignments for your child's classes on-line - is this an option and have you looked into it? Do you know specifically what her homework consists of? Doing a certain number of math problems, reading X number of chapters in a book, writing assignments, and so on?
Meet the teacher, but at this point in the year, this shouldn't be a sudden surprise. If there have been parent-teacher conferences before, that's when you could have been told what the goals for the class and what the teachers expectations were. Without more specifics, it's hard to judge if this is really excessive or not. At least she is trying her best and her grades reflect that.
I also know that my stepsons had homework during the week, but rarely on the weekends. What are her weekends like?