Most people find it difficult to concentrate on a task for too long. Many teachers apply this principle to their students, but fail to apply it to themselves. If you want to manage your time effectively it is important that you understand your own personal attention span clearly. It may vary depending on a number of factors, the most important being how tired you are and how interesting the task. However, most teachers have a kind of sea-level of work tolerance - it is this standard timespan which varies - down when you are tired, up when you are interested - and it is this timespan which you need to know. Trial and error is a good way of doing this, but it important that your current best-guess is entirely explicit. If you are a genuine 'don't know' at present, then assume that it is 20 minutes and modify that estimate based on experience.
Both knowledge workers and clerical workers typically require 3-5 minutes break between their 20-30 minute bursts of effort to remain near peak effectiveness.
Good time-management - the TM4T method, in fact - demands that you consider the work you do outside the classroom, and think about how you can break those activities down to tackle them more efficiently. Knowledge of our own attention span is vital in this. This means that you instinctively start to plan your work, and your time, in 20-minute chunks (or whatever your personal chunk may be). This will become automatic in the same way as planning your student's learning in 50-minute chunks (or whatever length your lessons are).
It is also important that you acknowledge any persistent personal bad habits: these may include behaviours such as procrastination, writers block, reluctance to finish, or impulsive multi-tasking (these personal aspects are sometimes called attention management).