The effective division of working time depends, of course, on the specificity of a given industry and even a given position. In my case, effective time division assumes that each shortest task allocates at least an hour of my time, and each hour of time requires another hour of conceptual work.
This means that during a typical working day I am able to participate in a maximum of four topics, which take up a maximum of one hour of working time - which in total fills up my eight-hour working time. To work effectively during these eight hours, I need an hour's break to be able to get up from my desk, maybe eat something, take my eyes off the computer.
Of course, none of us can afford to do nothing else, so organizing email, calendar and tasks is also a task on my daily chore list, in which I dedicate myself to completely clearing out my inbox and translating some messages into tasks. During this working time, I also have to accept possible meetings and write down the most important tasks for the future.
Similarly, too much distraction on even four serious matters during the day reduces efficiency. None of us is actually able to perform many tasks in parallel, although we all think that we do it more or less perfectly. In fact, our effectiveness is greatest only by limiting the number of different topics during the day, or by focusing on only a few important ones.
This means that the total working time in a given day extends to nine hours in my case, but it is not nine hours of work. To ensure the effectiveness of working time, some people set a seven-hour working day and this approach to planning working time is very common, among others: in Great Britain.