For those of you who may be unaware of 'flow', here is how it is described in Daniel Kahneman's Thinking Fast and Slow:
Fortunately, cognitive work is not always aversive, and people sometimes expend considerable effort for long periods of time without having to exert willpower. The psychologist Mihaly Csikszentmihalyi has done more than anyone else to study this state of effortless attending, and the name he proposed for it, flow, has become part of the language. People who experience flow describe it as "a state of effortless concentration so deep that they lose their sense of time, of themselves, of their problems," and their descriptions of the job of that state are so compelling that Csikszentmihalyi has called it an "optimal experience." Many activities can induce a sense of flow, from painting to racing motorcycles - and for some fortunate authors I know, even writing a book is often an optimal experience. Flow neatly separates the two forms of effort: concentration on the task and the deliverate control of attention. Riding a motorcycle at 150 miles an hour and playing a competitive game of chess are certainly very effortful. In a state of flow, however, maintaining focused attention on these absorbing activities requires no exertion of self-control, thereby freeing resources to be directed to the task at hand.
For me, I would say getting into a just difficult enough programming problem or working on a data analysis can put me in this state where hours can slip away in the blink of an eye. The same thing for a game of Civilization V can do the same thing for me.