As stated the Airac cycle is every 28 days. Every country promulgates and publishes any changes due to take effect within their airspace 28 days in advance. On the "Update" date, all relevant changes from every state's AIP are incorporated into a major database. A certain major UK airline runs two flight planning/navdata computers in tandem. One is live, and the other is a working copy.
All updated changes to standard routes as a result of the Airac cycle are loaded into the "dummy" system and checked for integrity. This can take an entire night shift for two nav data specialists, who will run each route on the dummy system to ensure that there are no route discontinuities. Once validated, the dummy system is copied over onto the live system, and engineering then updates each aircraft's FMS database in the manner descibed in the previous post.
Hope that helps.
Blue skies.