Richard Swenson, a medical doctor, popularized the concept of time margins in the mid-1990s. As described in Benny Tabalujan, ed. Leadership and Management Challenges of In-House Legal Counsel (LexisNexis Australia 2008) at 43, in-house lawyers would moderate some of the pressure on themselves, and be able to get some work done more effectively, if they purposefully created blocks of empty time in their calendars. Buffers of hours let you fit in unexpected calls, think through knotty problems, or simply change gears and recover a bit.
Deliberately unscheduled time, be it 30 minutes a day or a two-hour segment once a week, has benefits something like scheduling meetings for 50 minutes (See my post of Dec. 10, 2009: give yourself time between meetings.).