There are advantages in tackling work in bite-sized chunks.
For most of us, one three hour task achieves less than three one hour tasks, however we break it up. This tells us something important about time management. Tasks don't just happen. They are done by human beings whose attention span has a huge influence on their achievement. So, on one hand you should try to switch tasks every so often, to avoid getting bored or distracted. On the other hand, there are 'tuning costs' associated with most teaching tasks - the first essay in the pile takes much longer to mark than the tenth essay. It is therefore important to get the balance right and get a sensible answer to the question 'how long should I work on one task?'. There are swings and roundabouts involved, as we have mentioned - if your standard-task-length is too short, then you incur constant tuning or switching costs which limit your efficiency; if your standard-task-length is too long then your productivity will wane as you lose focus and energy. Most teachers find 20-30 minutes is about right; in the TM4T method we recommend a maximum of 20 minutes.
What this means in practice: you should choose your productive time-length; let's say it's 20 minutes. You should then develop over time a picture of what that 20 minutes means in practice. Ten pieces of GCSE coursework marked? Tick and flick two Year 8 classes? Plan a morning's lessons? In developing your Estimation skills, you should also automatically develop a feeling for how your chunks relate to the tasks you have to do.