No one but you can set your boundaries. Coworkers, bosses, employees, and customers all expect you to do business and work with them when they're ready by default. They are likely preoccupied with their own responsibilities and agenda. It's unusual for a colleague to guess that you're unavailable. That's why it's incumbent for each of us to outline windows in which we're available, and vigorously defend the time in which we're not.
Schedule Open Intervals
We all need time to handle unexpected changes, but we should strive to plan for miscellaneous work in addition to free time. Open intervals are flexible, but they can be focused on a particular challenge. For example, yesterday I blocked off 3 hours for walking, reading, and writing. Friday I spent an hour reading Kevin Kelly's New Rules for a New Economy. Blogging, reading and exercise are part of my open intervals.
Blocking off an hour of each day (minimum 30minutes) to unstructured activities, is the secret to always keeping one's mind fresh for difficult decisions (for more read about flow). This is a habit we need to continually practice, and a sanctuary we must nurture.
As your life and work path progresses, you will come to rely on the source of creative energy that open intervals provide. Expanding free time and open intervals in order to boost productivity will be a natural evolution of your day. It is also probable that your external responsibilities will rise in a correlated manner. Just as you are creating high value with your open interval, a deluge of business decisions will urgently require your attention. Make sure not to be consumed by urgent interruptions.
Open Intervals is a Long Haul Tool
For limited durations we can expend every quantum of our working energy. If we have constructed a work atmosphere that is aligned with our passions, we may not even distinguish between our work and leisurely pursuits. But over the duration of our lives (years) scheduling open intervals is not only rational, but healthy. Families, friends and activities outside of our direct work responsibilities (new hobbies/interests) will keep us centered for the duration. External activities also reduce the probability of circular feedback bias to our decisions.