I don't know if there's a "normal" or "recommended" length of term for a Contract such as this. You may be best off soliciting a proposed agreement from the management company, and hiring an experienced media law attorney to look over the contract briefly and advise you of the pros and cons of each provision, as well as any provision which is not present which may be beneficial to you.
Of course, without having heard the whole story and reviewed the relevant documents, I can't give you advice, just my general opinion. If you're serious, you should contact a local attorney, rather than relying on any opining on the Internet. Jason L. Eliaser is licensed to practice law in California (State Bar number 248394.) This is a communication concerning my availability for professional employment within the meaning of California Rule of Professional Conduct 1-400(A). Viewing of this post does not constitute the provision of legal advice and no attorney-client relationship is formed by viewing or responding to this post.
It depends on what your expectations of the manager are. For example, if one of your main reasons to enter into the management agreement is so that the manager will find a record deal for you, then you would want the management agreement to provide that if the manager does NOT find a record deal, meeting certain criteria, for you within a certain period of time (typically in the range of 6 to 18 months, but the exact number of months is typically negotiated), that you then have the right to terminate the management agreement at that point.
The above is not intended as legal advice and does not constitute the creation of an attorney-client relationship, as this forum does not provide for a confidential communication.
In California a personal services contract cannot be more than 7 years by law, but management contracts are usually much shorter - or should be. 2 years is typical and there are often options for extending the deal - sometimes automatically. You should be wary of automatic extensions. At least build in a way to terminate if you are not happy with the services. You should definitely have an experienced entertainment attorney review any contract before you sign.
Please note that this answer should not be considered "legal advice" and no attorney-client relationship is formed by answering this question. You should hire an attorney licensed in your state and familiar with the relevant areas of the law to conduct an analysis of your situation and provide you with fully informed legal advice. This answer is posted for general purposes only.