It depends. Some companies like to create what I refer to as the "knighted" project manager. They hand someone with no experience the PMBOK Guide and a copy of MS Project and tell them to go manage projects. If your goal is to become a good project manager, then depending on what degree you have and what kind of on the job training you will get from your company, 5 years is reasonable. You may also be eligible to the PMP Exam in that time which would be a demonstration of your attainment of a minimum level of PM experience.