You would need to extrapolate from what you've consumed after six months, one year, two years, etc. Some organizations find that their original projections were inaccurate, and that mail older than five years is not critical to the business. There is also the thought that data storage costs keep dropping, and that, in ten years, what you pay will be less than predicted.
Are you simply storing the mail for users' convenience, or do you also have compliance needs in mind? If it is for compliance, then your choice of a solution will need to take eDiscovery into consideration.