In one column put the date - in months, then in the next, your capital, in the next the interest (5%, or whatever, per year, divided by 12 for monthly), in the next, your drawings from the capital.
Make the capital next month be the same as the previous month, PLUS interest, MINUS drawings
See how long it takes to run out of money.
if you're smart, you can have the interest rate and monthly drawings in a separate row at the top, and link each formula back to it with a $ reference (Excell's shorthand for a fixed reference, not a floating one).
if you get REAL smart, you can also have currency inflation and factor-in that your drawings must inflate with time to balance the devaluation of the currency.
Retirement planning? I can send you some sample spreadsheets that I use for this, if you want. I can put these on an ftp site somewhere and post the link...