I've found that there's a "hump" where reading a book where I get bored and want to do something else after 15-20 minutes, but if I'm really engrossed and stick that out, I can find myself literally reading all night long, for hours at a time, until I finish the book. (I'm someone who tends to finish books in 1-2 sittings; I never really understood folks like my mom who'll read for 15 minutes a day and nurse a book for months.)

For that matter, programming's like that too. After a 15-20 minute chunk I'll usually feel a strong urge to go check HN (clearly, I'm in one of those now). If I can resist that urge enough, and have long enough blocks of time (challenging with a kid now), I can find myself coding for several hours straight until I finish the change I was working on.