When I signed up for Iris, they told me that my Iris registration would expire when my long-term UK visa expired (i.e. in three years). In the 2+ years since then, I've used Iris about once a month and have never had a problem.
The way they answered my question indicated to me that each person's Iris registration period is linked to the type of entry clearance each person has. I would take it to mean that there is no standard length of registration and it varies from person to person.