Really depends on what kind of productivity I'm trying to achieve. "Hard" tasks - some kinds of system design, and the long tail of certain kinds of debugging

Really depends on what kind of productivity I'm trying to achieve. "Hard" tasks - some kinds of system design, and the long tail of certain kinds of debugging - I may only squeeze out 1-2 hours per day of effective work. OTOH, there's plenty more "straightforward" tasks which, if not interrupted, I'll be productive for the entire time with.