As a general practise a content management system should be regularly maintained each year as part of routine support and maintenance work. Upgrading incrementally keeps the CMS current, performant and reliable.
There should be a review each year to assess the benefits from an upgrade, if within that the year there are no real platform benefits then it should be done by year two as a maximum – anything longer than this starts presenting challenges.
Finally, it is cost effective to upgrade incrementally to avoid the need for a more complex migration, restructuring and re-architecture to bridge the gap between different versions.