Time-boxing has been somewhat successful for me.

(Time-boxing, which involves giving a task a certain period, became famous thanks to people like Cal Newport and Mark Forster.)

When I try to time-box and fail, it's usually because I go over the allotted time, usually by a lot (very seldom by a little, but that's for another day).

One piece of advice I've heard is to multiply the time you estimate an activity will take by 2.5 to get the actual time it will take. According to Hofstadter's law, a task will always take longer than you give it, even if that time is 2.5 hours. And yet, the 'why' question still remains: 

Why do I consistently (and somewhat irritatingly) underestimate the length of time that anything will take?

In my opinion, the distinction between Calendar time and Labor time is the "why."

Calendar Time or Scheduled time

I cannot work non-stop on a lengthy task. It depends on the task, but generally, for me, the time is just under an hour or so. For you, the time may be longer or shorter, but at some point, breaks are necessary.

Flow is not an infinite state, no matter what we do. Some studies suggest approximately 50 minutes is the ideal duration, although I have seen other research that suggests various times. For instance, the Pomodoro technique made 25 minutes famous.

If you are interested in reading more about flow and breaks, then a book I recommend is The Breakout Principle, by Benson and Proctor (2003).

Breaks for restroom use, eating, sleeping, vacations, and contemplating and digesting. All are accounted for when calculating the task's completion time in the calendar or gross time if you like. Different things to factor in include uncontrollable factors like waiting for other people. For instance, if I need to order paint to paint a house, I can't begin until the colour has arrived. Because of this, project management can be an arduous task. 

Labour time or Work time. 

Without any pauses, this is how long the job will take. It might be compared to "net time." Don't think about how long it will take for the paint to arrive. 

An Illustration

I have to grade 30 assignments. I have a maximum of 15 minutes for each task. That translates to 640 minutes, or less than eight hours, of "labour time." But given the calendar time, it might take ten days to finish this task. It is difficult to grade more than 4 or 5 assignments per day; by the time the 6th assignment arrives, my head is like a pickled egg. Additionally, I enjoy going back after leaving them overnight. 

How does knowing all this help? 

I believe that simply being aware of the distinction will help to timebox. If I timebox labour time (which is frequently how I operate), I never finish in the allotted time. It's disheartening, to put it mildly. It becomes a far more useful tool for prediction if I timebox to calendar time because I am more likely to meet the deadline. 

Knowing the difference between both times means I can assign calendar time as a maximum and labour time as a minimum. Unfortunately, it will take longer anyway, but that is just how it is.