Estimate the amount of time of your savings will last based on what you have to start, your monthly income deposits, and your monthly expense withdrawals. For a similar but more detailed calculation, use the Future Value of an Investment Account Calculator.
This calculator assumes that you have a retirement savings account and you receive a monthly fixed income from, let's say, social security. Also, you have monthly expenses that are greater than what you receive from social security so you have to withdraw from your retirement savings on a monthly basis. You want to know how long your savings will last you.
Let's say you have a starting balance of $75,000 at 4% interest and you know your monthly income will be $1,800. You want to know how much you can withdraw on a monthly basis to make your saving last at least 10 years. Try entering expenses as $2,800 per month and you'll see that savings will last 8 Years and 1 Month. Keep lowering the expense amount until the time is what you want. At $2,550 the savings will last about 10 Years and 1 Month.