Writing character backstories is great practice for writing in general. The character sheet is background research, which you use to answer a few prompts:
1) How did your character acquire his skills?
2) How does your character behave under ordinary circumstances?
3) Why is your character adventuring, given the high mortality rate?
As long as those questions are answered, getting the backstory to the DM before the campaign begins is more important than writing a high-quality backstory--another reason why it's good practice for me. You can go into as much depth as you like on any of the prompts, and naturally backstory length varies widely. A good one can be a paragraph or twenty pages, but the shortest are more likely to be uninformative, the longest unreadable. The DM's got enough work; saddling him with a novel-length character description, or forcing him to build your character's identity himself, is inconsiderate.
Here are a couple of backstories that I've written in the last week. The first is a work in progress, both because I want to make it longer and because the character is in an ongoing campaign. The second was a thought experiment.
Sharran was born the scion of a noble family, descended from celestials in the service of Heironeous, and thus naturally devout followers of that god. The lion's share of the familly served in the capacity of Heironeus' clerics and paladins, and it was expected that Sharran would follow the same route. But Sharran was quite unlike his relatives. His short, wiry frame and scrambling approach to life contrasted sharply with the contemplative and courtly habits of his tall, graceful siblings. Rumors that Sharran was illegitimate added to a general atmosphere of unease concerning the young heir, which served to isolate him.
Sharran objected to the righteous fervor and militaristic demeanor of the Tarciels, which he believed insulated them from the rest of the world and subjected them to an unhealthy stasis. When it came time for him to choose his path, he wanted nothing more than to shake things up. As he was presented with the ceremonial sword and shield that represented the choice between clergy and the knighthood, inspiration descended upon Sharran. Taking an ordinary stick that laid nearby, he dealt a mighty blow that broke the sword and shattered the shield. Then he strode out the door and out of the city, for good.
It has been many years since that day. Sharran has come to the understanding that Fharlanghn the Dweller, god of roads, is the source of his powers. But he still does not fully understand how he came to be a druid of the traveling deity. For one thing, he cannot find companionship with animals, a central tenet of druidic teachings. Perhaps it's a parting shot from his family and their Outsider blood; but then, how is he to progress towards enlightenment? (How ironic that the blood of angels should be such an impediment.) How can he reconcile his philosophy of change with the druid's focus on balance? Sharran travels in search of answers. But he may have to put his personal dilemmas on hold in the face of a plague that threatens all.
Jenaya grew up on the streets of Skyfane City, carving out a life for herself outside the law. And she was good...very good. She came to view crime as an art form, from the skillful choice of mark to the evasion of the Man afterwards. Her devotion to the art form soon exceeded her devotion to crime--though she was hardly averse to breaking the law. She began to take on higher-profile targets, letting her whimsy dictate her choices. The day after breaking up a crime ring before the law arrived, she would thieve a priceless treasure from a well-guarded aristocrat. The only constant was the fox mask she left wherever she did her daring work. Rumors spread about the Fox--a man with mastery of illusion magic, a sorceress with great powers of seduction, a quickling with an uncommon interest in human affairs. Jenaya let the rumors take hold, even added some of her own--and quietly laughed whenever she heard them on the street.
Even so, someone had the wherewithal to track her down. She woke up one night to find herself surrounded by hard-looking folk. A man, their apparent leader, sat at the foot of her bed. He introduced himself as an enforcer of the Nightsong Guild, a powerful player wherever there was play to be had. Jenaya had been stepping on more than a few toes, and so come to the enforcers' attention. The man offered her a mutually beneficial arrangement. Membership in the Nightsong Guild would give Jenaya greater resources with which to pursue her antics, and legitimacy in the underworld of civilization. In return, the guild would benefit from the notoriety of the Fox, and occasionally ask her services for particular tasks--and take a cut of the Fox's prizes, as well. She could refuse, of course, but then she would risk making the guild her enemy--and they had amply demonstrated their ability to track her down.
Jenaya accepted. Privately, though, she saw an opportunity to outdo all her previous work. How would it do to trick the underworld's most famous tricksters? As she trained in the team tactics that characterized the Nightsong Guild, she also began her preparations. She gathered information about the guild's leaders, its network of operations, its allies and enemies and triply turncoat traitors. And she began scheming to take down the guild from the inside.
But she was found out before she could set her plans in motion, and the entire guild was mobilized against her. She escaped by the skin of her teeth, and now she's on the run. The Nightsong Guild even took the unprecedented step of releasing information on the Fox to the law--betraying the underworld's cardinal rule. With nothing left to Jenaya but her devotion to trickery, she constantly heightens her skills in order to avoid the authorities by day and the assassins by night.