On the other hand no backstory is not the answer. Without some form of a grounding in the world your character might as well have popped into existence out of nowhere. If you only want to play in a hack and slash game this may be alright but even then what about when you go to town? Without a backstory you don't have any friends, you don't have any connections, you don't even have a family!
My answer to how much is good enough is in two parts. First a paragraph describing the character, where they grew up, what their job was, and why they decided to start adventuring. Nothing much, just enough to have an idea of what they did before in their life and why they decided to go adventuring.
The other thing I require is a list of close family member such as the parents and any brothers or sisters. The list will have for each person their relation followed by name and finally occupations. As an example:
|Older Sister||Jessica Smith||School Teacher|
|Younger Brother||James Johnson||Guard|
With just those two things all kinds of ideas of what your character may be like can spring up. Both parents are farmers so you probably know how to handle farm animals and what the weather is like from your area. If you lived by a lake or some other big water feature you probably know how to fish and maybe swim. Your sister is a school teacher and because the parents are farmers that probably means one of those small schools for the local farm children, you may even have had her for a teacher depending on the age difference. If your a fighter type then the Younger brother being a guard probably means he looks up to you but if your a thief then he may actually dislike or even hate you and chose to be a guard to try to catch you. It is amazing what you can extrapolate form just a paragraph of info and a list of names.