Before session 0? None. It's group policy. No character creation before session zero. Session 0 is where DM gives overview of campaign, setting, themes, style, type of characters that fit/don't fit in the game, house rules used for pc generation, house rules for future campaign. Those informations are crucial to create characters that fit into the game and mesh with each other.
After S 0, usually couple of sentences and that's it. It depends on the starting level. If we start in tier 1, very minimal backstory. And i try to stay away from convoluted complex backstories. Couple of sentences about his upbringing, how and why he got to where he is now in life.
For characters in tier 2 or 3, we, as a group, create parts of backstory together, since those are characters who are already established figures in the world.
Both as a player and DM, i don't like 5 pages long backstories, especially about low level characters. You are nobody in the world, hero of your village/ neighborhood at best. Keep it to paragraph or two max, bullet points are preferred. For higher level characters, shared background is must. How, why, and when did party met, what did they do together, what are their goals etc.
Now, for my VtM games, i want more backstory. Specially since i usually run games with characters of 10-11th generation with 150-200 years of active experience. I wanna know what you did, who are your allies and enemies, what schemes are in motions etc. General rule - older and more experienced character is, more info i want. But i appreciate when players keep it concise. Even then i don't like reading 5 pages. Half page bullet points with most important highlights of your past is enough.