This is a practical question and I love it.
Talking about money, it is one very powerful commodity in our world. Even though it's just some kinda paper or coin with the name and face of a dead human, it is literally the reason why some persons still keep living.
The quest for money can not be overemphasized as even a little kid desires some money to be able to afford chocolates and some toys.
Now this question says; how much money would I need to leave comfortably FOR THE REST OF MY LIFE?
I bet you know no one knows exactly how long he/she is going to live and what challenges might suffice along the line,so it is reasonable to say that there just can't be any sort of budget to sustain an individual for lifetime. Every body has different needs and wants, but then, when comfort is of utmost concerns the list of needs and wants seem to only increase.
It is true that humans differ, so what satisfies Mr A might just be an appetizer for Mr B.
Living comfortably can mean a lot here in my country Nigeria. One can say he/she is living comfortably when they possess a duplex, well furnished with all what have you. They must also have at least two cars, so in a couple scenario the wife drives one while the husband the other. They must also have in the their possession big businesses that keeps pumping in money into their bank accounts. Finally, they should have a big fleshy and nice dog breed. If all this in place, you can confidently conclude that they live a comfortable life.
On the other hand, if your question is directed to only huge physical cash , then we can reason from this angle..
Practically, money is very volatile and can lose it's value within the twinkle of an eye, so it is reasonable to note that if money isn't invested in a profit making venture then it can vanish even before you know it. Take for instance, you have a Billion US dollars to live life with and you take no initiative to invest part of it in a business, you only keep spending without returns, trust me you wouldn't know when the Big doh finishes.
It is also true to a some extent that youths(18-35) spend a lot of money basically on fun things and perhaps gadgets, reasons being that most heavy responsibilities may not be present at that moment but when they get to age say(40-60), then the expenses become even more because at that time they would have a lot of bills to pay and children to train. So you see, adults might be actually spending more money..
But in old age, when virtually all heavy responsibilities have been taken off, one would spend less money on upkeeps. This doesn't mean that if investment opportunities arise, they should be looked down upon. No one can actually tell what the next days economic situation would be so it is advisable to always invest our money on legitimate profitable businesses.
Finally, i am of the opinion that no amount of money(fixed) would be enough to earn me a comfortable lifestyle for as long as i live..