I think you want the unit I always round up to: sitting-in-your-source-code-folder-unviewed-for-a-year-with-all-intentions-of-finishing-it-though-really-just-avoiding-it-because-you-know-it-is-way-too-large-to-really-finish-to-the-degree-of-polish-you-initially-envisioned.

Really the point is to keep yourself honest and realize that things tend to take a lot longer than you'd expect.

Also, if you are scoping a project out in terms of years you probably want to break it down into smaller features and implement them one-by-one.