Where the target comes from
The common benchmark is the 4% rule: if your money stays invested, withdrawing around 4% a year has historically left the pot intact. That is why a year of spending gets multiplied by 25.
It is a reasonable starting estimate, not a promise. Markets rise and fall, and inflation pushes your spending up every year. Treat the number as a destination, not a date.
What moves the result most
Not the return percentage — the monthly contribution. Returns are outside your control; what you put in is not. Change that one field and watch how many years disappear.