Ezrak,
Unfortunately, there is no single "right" answer to this question, because the reality is that how much you need to save depends on what your goals are in the first place. The amount of money you need to retire spending $3,000/month for 20 years at age 70 is quite different than what you need to spend $5,000/month for 35 years at age 60!

That being said, there is a growing body of research that suggests that you can spend about 4% to 5% of your assets (1/20th to 1/25th) when you retire, and adjust that amount each year for inflation, and never need to worry about market volatility. On that basis, if your goal was to have $4,000/month, or about $48,000/year in spending, you would need 20-25 times that amount in retirement savings, or about $960,000 to $1,200,000. However, if you have Social Security, pensions, or other income sources coming in, then you will need less in assets since you only would need to generate a portion of your income from withdrawals. For instance, if you had pensions and Social Security adding up to $2,500/month, then you might only need to support $1,500/month in spending from your assets, which is $18,000/year, and would require 'only' $360,000 - $450,000 in savings.

Remember that you need to account for changes in inflation over time, how long you plan to retire, and to incorporate all of your goals, but hopefully this is a helpful starting point!
- Michael