You'll find out soon enough. Grad students generally aren't stressed "all the time," but tension and deadlines run high a lot of the time. For a comparison:
I took 6 classes (24 credits) my last semester of undergrad (I needed to graduate in 3 years)
One of these 6 courses was my senior seminar in which I had to write a 30-40 page honors thesis
I worked 3 jobs (time fluctuating between 30 and 60 hours per week)
Plus I volunteered my time as a UG-TA (usually grading papers)
*Even with that workload, I had more time to myself as an undergraduate than I do as a masters student taking 9 to 12 credits (3 to 4 classes) a semester, writing my thesis (140 pages/40,000 words), TAing (instructor of record for one 100-level course and one 200-level course), being a member of a couple curriculum development committees, presenting at conferences, and other odds and ends expected of graduate students. Time management, as everyone else is saying, is essential. Here's my order of priorities: 1) teaching, 2) coursework/thesis, 3) professional development (committees, conferences, random certifications, etc). It's worked for me so far. I'll be done in May with a terminal masters.
Time breakdown (average week):
In class: 9 hours
Homework/thesis: 30 to 60 hours
Teaching/grading/office hours: 20 to 40 hours
Sleeping: 40 to 56 hours (when you have multiple big/important assignments due, sometimes you just don't get to sleep at night)
Left over: Readings, committees, conferences, parties
Also, graduate school is what you make of it. If you don't feel challenged, there are usually plenty of opportunities to work yourself harder: internships, assisting profs with research, mentoring, joining/running graduate organizations, trying to publish (important if you want to continue on to a PhD). Coming from Carleton, I know how strenuous undergraduate programs are. But in general, no undergrad will hold a candle to the type of work expected from you at the graduate-level. (Students who feel their graduate work is too easy have either built up their expectations beyond reason or are in the wrong program or at the wrong school).