Hoo boy!
One time I wrote a 66,000-word cross-referenced alphabetical encyclopaedia.
I spent months coding the star system generation sequences of GURPS Space (4th ed.) as an Excel workbook with no macros.
I constructed a life table for the population of Imperial Direct Jurisdiction taking into account the recruitment of Imperial servants from the colonies over 120 years. Also, a life table for the Imperial Family, taking into account a diminishing rate of exogamy.