I did 18 in a 5 week summer session but it was the same: grad and undergrad at the same time. I had a wife and 6 month old kid and i sent them home to her mother for the whole 5 weeks and basically just ate speed and went to class and studied for the whole time. It really sucked but it was the only way I could get my stipend the next semester and I needed it so my wife could stay at home with the baby while I was in grad school. The postscript is that I got divorced in my 2nd semester of grad school!