I think the answer is going to be different for every writer. Sometimes I get a similar question, “at what point does knowing my characters make writing about them boring?” That too is a hard one to answer.
In my experience, there is a huge difference between knowing just their personality and what will happen to them. When I first start writing, I don’t know my characters at all. I might think I do. Even when I plan out who they are going to be, I can tell you, I don’t know them until I’ve written about them. I figure them out on the page. One I get a sense of them – their thought processes, their personality, and so on – writing about them is easier (and sometimes harder).
Really though, at least for me, there is so much thrill in learning about your characters as you write. You discover something in your writing in moments between characters. It’s exciting. And sometimes, by leaving room for this kind of discovery, you might find that the character you worked so meticulously to create is not a character who is going to do the things you’d planned on them doing down the road in your outline. While they were on such a neat little trajectory, in the process of discovering things about them, they might change their mind or their outlook. Characters change a lot and sometimes very quickly.
I tend not to worry too much about backstory. I won’t say it’s not important or significant, but it only acts to try to explain who the character is at the start of the story. Sometimes backstory can act a little to causally. Like, the character is this way because his mother ran away with the family dentist, his father was an alcoholic, and even his family dog didn’t want to stick around. Really, those things might have happened during his childhood and shed light on what happened to important figures in his life, but it says nothing about who he is.
When a character first enters the story, his backstory might not matter that much. You don’t need to “explain” characters. Sometimes they are just the way that they are. Don’t focus on explaining their life leading up to the story, but who they are when the story starts and what happens to them as they face conflict.
Early drafts of any writing project are always going to be about you learning who your characters are. You won’t know them at first. It takes time to figure them out. How you do it is up to you. Some people plan and outline, other people just write. Everyone works differently. It’ll be a matter of figuring out what method works best for you. Dare to try things out.