1 Charity Collector Guy: I’ll add up the total owed and you’ll be issued a back taxes bill. You’ll have 30 days to pay, with interest.
2 Head Death: HOW CAN YOU SUM 21 MILLION HANDWRITTEN NUMBERS? THAT WOULD TAKE A MORTAL YEARS TO DO.
3 Charity Collector Guy: Fortunately we’re in a realm where the normal rules of time don’t apply.
4 Head Death: WELL, HELL.
4 Charity Collector Guy: Not quite. But close.

Assuming you can add at an average rate of one number a second, without pausing or resting, it would take you just over 243 days to add 21 million numbers. Assuming you'd be more comfortable sleeping and eating, and so only working say 8 solid hours a day, it'd take you 729 days, working 7 days a week with no time off. Which is just a few hours less than two years, as it turns out, coincidentally.

I reckon you'd be pretty sick of it after two years and would slow down at some point, so the Head Death's estimate of "years" is right on the money.