My preference is for a back story to be no more than a couple moderate paragraphs laying out who you are, where you came from, why you're out poking gnolls with pointy things instead of staying safe at home and how you acquired your initial gnoll-poking (or toasting, etc) abilities. As a level one character, you're both not an exceptional thieves guild master or general but you are a step up from the common folk so a little story embellishment is appropriate. Add in a couple potential hooks that the DM can use in the game but nothing that leaves a weird hole if the DM doesn't use it (i.e., "I've been haunted since the death of my sister" vs "Every waking moment of my life is dedicated to avenging my sister... except right now where we found a cool skeleton tomb")

You're certainly welcome to describe your entire family tree for seven generations if you feel like it helps you get more into your character but I don't want/need that so keep it as a solo project.