With the ups and downs of today's market, how will I know when I can retire?

At age 31 it is not feasible to know precisely the year one can retire. This is because of the volatility in the stock market. Moreover we can make some assumption and make projections for needs post retirement. Throughout your working years periodically compare projected balances to actual balances and modify the plan as needed. One can pick a desired age as a retirement goal and adjust savings to cover retirement expenses. If it appears that the current income does not allow you to save what is required to retire then you need to adjust your goal as needed. For example, retire later or reduce expenses now which will free up some money to put towards retirement.