This is doable, if we are talking about intranet. Just hook something in your web server. This code shall be made aware of each user request, and has a data structure to store the time of the last request from each client, so it can calculate the intervals between requests. But obviously that interval is not precise, as you cannot say that the user 100% focused on a page before he/she requests the next one. (Internet would also be fine, as the users in an organization, most likely would access internet through some sort of proxy.)

This could also be a small proxy program on the client side, which captures each user request before it forwards the request to server. In this case, it does not matter whether we are talking about internet or intranet.