It depends on the staff system you're going to learn. Some use a "long staff" that only reaches to shoulder height, while others use a "long staff" that's goes up to your fingertips when you extend your arm over your head, and I even know of one system that uses a 12 foot plus staff!
Consult with your instructor to find out the right length for what you'll be taught. If in doubt, 6 ft or so is a good starting point, typically. Error on long, because you can always trim it -- but it's hard to add length!