It really varies. Lectures- more than history/english types, less than economics.
External workload- depends entirely on how fast you read and comprehend, and how easily you grasp and retain arcane and convoluted reasoning. Also your memory, and whether your lecturers are any good.
If you're lucky, you'll get by on an hour every day or two during normal term time, recapping all the lecture stuff and noting your books up. In exam time you don't sleep.
Two things to bear in mind- everyone who gets a first works their arse off. Coasting like I suggest is only good for a 2.1, and a middling LLM. Second, different areas of law require radically different amounts of work. I didn't look twice at contract or commercial after lectures until revision time, but equity and land had me clocking up facetime in the library like I was being paid by the hour.
Just never go to lectures drunk or hungover, don't miss any, and make sure you've got a noting system that means you get every important point noted while still actually really listening and thinking. Also take active part in workshops- they exist for the staff to teach you how to boss the exams.