Nimrod, I think you are right - it is heavily dependent upon field. I am in science (microbiology mainly) and while I was a tiny bit slack in the first 2 years (not mega-slack though), I worked pretty damn hard for the last 2 years.
In the last 6 months I was trying to finish benchwork, my boyfriend had already moved 800 km away to live (with our dog and all our belongings...sob), time was running out, plus my scholarhsip had run out 6 months before, I was doing 10-12 hour days in the lab every day - Saturday and Sunday. I survived only because my mother-in-law brought me food parcels - she is a great cook!
DJW - you must be really efficient when you work and you must be really organised. You are lucky. No 5 hour days for me in the last 2 years. I guess not having to work with fickle bacteria would help.