The four percent rule is a rule of

The four percent rule is a rule of thumb used to determine how much a retiree should withdraw from a retirement account each year. This rule seeks to provide a steady income stream to the retiree while also maintaining an account balance that keeps income flowing through retirement.

By analogy compute any percent for any number of years! 

Just divide 100 by the number of years you want spending

 some amount and then use the perceentage computed annually! Simple right?