npub1fgle6lnth5pces3v5hayhvdsaqg7r842gxhg9fh5a7l66jm2qp3q0w4jl4 (npub1fgl…4jl4) That's really an individual decision, but I can share my approach.
Typically, as soon as I have enough about the character that they can function in the world I start writing, adding to their background as I go along. No matter how much I do beforehand, when they actually start living a lot changes &/or is added.