When devising a character backstory, how much detail is "enough," for you to get a good sense of them?
It’s easy to get wrapped up in a character’s backstory just like it’s easy to get wrapped-up in world building. Both are necessary story elements, but there is an element of diminishing returns after a certain point. Like world building, the character’s backstory is only interesting if it serves the story.
I tend to think of character backstory as a path; what did they do to get them to where the story starts? Not all of it is relevant to where the story goes; Robert Parker’s Spenser spent some time in the Army and some time as a state trooper, but none of that is as important as his years spent as a boxer. Mainly because he gets into 2-3 fistfights per novel. It helps that we know he used to be a cop, but as readers, we don’t need to know anything particular about his time as an officer of the law.
Really, it comes down to what story are you trying to tell, and what backstory elements support that story. Some that might really important—like Spenser’s first name—don’t actually matter much at all.