I work at a school and we've decided to ditch our physical textbooks and move to a digital textbook (i.e. webpages). One reason to do this is the fact that this publisher updates the material constantly (it's much easier because it's a webpage instead of a physical book). Last month, the Charlie Hebdo attacks were included in their text.

This is one publisher though. The offical government standards haven't changed. They're formulated in very broad language anyway. For the period after the Second World War, we have to cover 'decolonisation', 'the Cold War', 'cooperation within Europe', 'increasing prosperity and social-cultural changes', and 'the development of pluriform and multicultural societies'. You'd be able to fit anything which happens these days somewhere in those last few categories.

To get back to your question, there's no standard gap of years but there is a delay based on practicalities. Firstly there's the question of how quickly publishers can update their material (quite quickly in the case of digital material, less so where physical books are concerned). Secondly there's the question of how often standards are changed updated. This will vary per country/state. A gap of a few years won't be uncommon.