I think depending on your major drastically varies. I know that I almost lost it at least once a week because I was so stressed during grad school. A typical schedule for me was 3-4 classes per semester with papers for each due every 3-4 weeks, one year, I did a 21-hour per week internship and then worked 35 hours in the evenings/weekend. I managed to get through it though (it was definately a miracle) and now I am an MSW.
I do have friends who have gotten Master's in business-related fields and their programs were not as stressful. (I think this may be because internships weren't involved, no papers, etc.)--don't quote me on that though.....
OVERALL I would have to say the level of stress experienced for any Master's program depends on the individual and the level of their stress-threshold. This is different for everyone.