If you are the typical household making around the median wage and plan to retire at the normal retirement age, then this simplified rule of thumb is a good place to start (certainly better than guessing!)
Simple Method to Calculate Recommended Retirement Savings by Age
Of course there are a lot of assumptions involved, but as a rule of thumb, it is a great place to start. Most people do not drastically alter their lifestyle after retirement, so basing it on your income is a reasonable assumption. And this method gives a quick and easy method to calculate if you are on track or not.