Here’s David Allen’s answer, from his latest newsletter:
Q: How much time do I need to get my email inbox to zero?
A: For people who have 50+ emails a day, I’ve noticed that it takes an average of about 30 seconds each to process (decide what it is, delete it, file it, respond to it quickly, or defer it to an “action” folder or list.) For someone with 100 emails a day (more and more common) that’s 50 minutes just to get through a day’s email load. That doesn’t count all of the other input you get as well, including phone calls, voice mails, conversations, and meetings.
A typical professional these days must factor in at least an hour a day and an additional hour at the end of the week (for a Weekly Review.) And not as “It would be nice if I could…”—but as an absolute requirement to manage their life and work with integrity.