Deciding when to retire requires a taking holistic look at your finances. How much money you'll need and how long it will take to save that amount depends on your lifestyle, current assets and long-term goals. To develop a realistic savings target, you first need to estimate your expenses in retirement. Then you should consider the different sources of income you will have and the payments you can expect from each at different ages. Finally, you'll want to ensure you're prepared for the unexpected, whether that's a major expense or just a long life.