I teach a 5 hour IELTS prep class on a regular basis. I believe there are studies that show that 70 minutes is about as long as any human being can sit in a classroom and learn effectively at any one continuous stretch. In that 5 hours, they get two 15 minute breaks and one 30 minute break halfway through. Two hours? If your students are paying for their class, I doubt they'd appreciate losing 33% of what they're paying for.