Monthly Income Needed?  To do everything you want, how much monthly income do you need?

Years From Now? How many years from now do you want your income to come completely from investments, so you have the choice to work or not work? 
(If you're 40 and want the choice at 60, that's 20 years.)

Future Income Needed. Years from now, things will be more expensive.  Based on monthly income, 3% annual inflation, and number of years from now, you'll need this future income to have the same buying power as the monthly income would give you today.

Years For Income to Last?  How long do want your investment income to last?  Probably the rest of your life!
We have to define that in a number of years. 
(If you're 60 when you stop working for a paycheck and you want the income to last until you're 95, that's 35 years.)