This questions has been vaguely on my mind for some time now. Today, I tried to pin it down more precisely.
Headline result: I optimistically have 103,271 productive hours remaining. Pessimistically, I have 78,972 productive hours remaining. 
See this spreadsheet for methodology and details.
Rough definition of "productive hour": An hour that results in substantive progress on a work product, a personal project, or a personal goal.
: Assumes 47 years of working 5 days/week for ~9.3 hours/day optimistic
or 7.15 hours/day pessimistic, taking 5 weeks off each year, followed by 20 years of retirement (optimistic) or decline (pessimistic). Further assumptions are documented in the spreadsheet.
[rereads: 2, edits: added a comma, added "a" to final clause of final sentence]