3-21-0 is a popular system for batching email processing in time blocks.3 The method involves processing email in three sessions per day, 21 minutes per session, to reach inbox zero. By batching your email into three processing blocks, you’ll limit distracted attention and protect non-batching times for valuable “deep work” activities. Know this: a proven process like 3-21-0 for email (or anything for that matter) is better than no plan at all.
Source: Ben Meer