The answer to that question ranges from "now" to "after she's fully grown" to "never", depending on who you ask. There has been research over the last decade or so that indicates that leaving them intact through at least a couple of heat cycles has a positive impact on several health issues, including cancer and orthopedic problems. However, the more heat cycles a bitch goes through (especially if she is never bred), the higher her chances of developing pyometra, a uterine infection that can be life threatening.
The main question you need to ask yourself is "can I ensure that she won't accidentally get bred if/when she comes in season?". If the answer to that is yes, then you can leave her intact for a while. If the answer is no, then having her spayed before her first cycle would be a good idea.