Instructor Matt Izzo with his student Vince and the Solo Flight Certificate

There are a number of guidelines set forth to help the instructor evaluate a student's readiness to solo, including knowledge requirements (which may be tested orally or by a written test), and the demonstrated ability to perform certain maneuvers, but in the end the determination rests with the instructor.
Thus, the decision is a subjective one, and there is no set number of flight hours which define when a student is ready.