The 3 latest versions are supported at any given time. This means that at the time v8 is released, v8, v7, and v6 will be supported and v5 will reach its end of life. This means that, if you are using v7 right now, it will be covered until v10 gets released. This corresponds to about 4 years of coverage.
With Odoo Enterprise, you have the choice of upgrading at every version or to wait and upgrade by steps of 2 or 3 versions at once. In fact, you even have the choice of upgrading from an old version to another later old version if you want to do that.
More info at https://migration.odoo.com/