I feel like its difficult to quantify for jobs where you're being paid to think. Even when I'm goofing off, the problem I need to solve for the day is still lingering in the back of my head somewhere. Actively squinting at it doesn't seem to make things go any faster and when I do return to work it's usually to mash out reems of code after letting it stew, but yes, the actual amount of time I'm fulfilling my job description is... less than my working hours.