Like everything else AIS worldwide, they're changed on a 4-week cycle called the AIRAC cycle. That doesn't mean that charts are valid for 4 weeks, only that the changes take place on specified dates:
9 June 2005
7 July 2005
4 Aug 2005
1 Sep 2005
29 Sep 2005
etc.
You'll find the AIRAC cycle number (e.g.05/2005) in the bottom right corner of the chart.
That's not to say that charts can only be updated with the AIRAC cycle, but the vast majority are.
More details at AIP GEN 3.1 para 4