Retirement
Ask twenty people what retirement means and you'll get twenty different answers. Retirement is about freedom and flexibility. It is defined by being able to stop trading time for money.
When can I retire?
If you've saved enough to spend the rest of your life, without needing to work a single day, while you have your expenses covered, you're good to voluntarily retire. Think of retirement money as a bucket of money you saved over years. Some people wait for the whole bucket to be full before taking retirement, while others may retire sooner (say, at 80% bucket filled) so they have to work only for 20% of the money for the remaining lifetime.
How much should I save?
This is really subjective, and depends on your lifestyle and family needs. There is no real math to it, but general guidelines, that vary from person to person. The online calculators may show absurd numbers, some too high, others too low.
No one knows how long they'll be around in this world or how they'll leave the world. Get a sense of your monthly expenses when you were to retire, and multiply by the number of years you expect to live at most. This is the best you can do, and have peace with it.
Where to keep the money?
It matters where the money is. It's a no brainer that the lifestyle of a person with all their retirement money in 401k will look very different than a person who has all of their money in a bank with taxes paid up front.