As a judge, I look for about 3 - 5 steps back, a pause, and then the same number of steps forward. The back should be done smoothly and willingly, with a minimum of pressure on the bit, no head tossing or mouth gaping open, etc. (And I love it when the driver glances over his/her shoulder before asking for the back.)
Contrary to one of the comments above, the back does indeed carry a fair bit of weight in the judge's decision on placings, especially in Youth and Ladies classes where excellent manners are of utmost importance. As someone else noted above, a poor back can make the difference between winning a class and not placing at all, and is often used as a deciding factor between horses that are very close in standings after their rail work.