I made detailed briefs for the first week of classes. After that, I realized that all the professors that say they require briefs and all the upperclass students that say you should do them for at least first semester/month/til you get the hang of it were wrong (at least for me).
Briefs are purely to save face in class as far as I'm concerned. If you have a decent memory, you can simply highlight key facts, the rules, etc in your casebook and use those as your starting point to jog your memory of the case when you get called on.
If you are concerned about having briefs at the end of the semester for exams, don't be. When exam time comes, all you really need are the rules and any key reasoning your prof emphasized. Reading through briefs is usually overkill and you don't really have the time anyway.
I just kept a running list of the cases I read in each class with the rule and any reasoning the prof emphasized (usually no more than a few lines per case). Then as finals approached I took this list and used it to make an outline -- supplemented with a commercial outline and a reputable upperclass outline from the year before. By keeping track of everything covered in class, I didn't miss anything from class that the commercial outline or upperclass outline didn't cover, and I didn't include anything in my outline that we didn't cover in class that was covered in last year's upperclass outline or the commercial outline.
I know classmates that wasted a ton of time briefing cases first semester and even some all year. They never seemed to have enough time in the day. I didn't feel nearly as stressed because I wasn't wasting a half hour per case putting together a brief that recited exactly what was in the casebook and would be said by the prof (or socratic-ed out by the prof) again the next day.
Summary: Briefs are overrated. Don't feel pressured to do them if they're taking up a ton of time just because you think that's what you should be doing.