Brief as long as you feel the need to. Once you find that you're hitting most of the key facts, the main points of reasoning, and the holding down, I'd make the jump to bookbriefing + decent reading notes.
I briefed until midway through first semester, and stopped when I felt comfortable. It's important not to get too bogged down in briefing, and remember that it's only one tool. If you feel you're benefiting, keep doing it. Once I jumped to book briefing, I still took pretty detailed reading notes in outline form (pulling rules and main concepts from the cases and notes), and combined with my classnotes, it was comprehensive and easy to turn into an outline. That worked for me pretty well, so take it FWIW.