Updates are a continuous process with no set standard and could be needed at any point. There is no standard to updates because they happen when a developer finds and solves a vulnerability in the code that makes up your site.
There are two main pieces of your site which will have updates: Modules (or Plugins) and your sites Core. Both pieces are important to keep up-to-date because they work in union to create your site. If one is not up-to-date then it may no longer be compatible with the other.
We offer a POM plan (Peace Of Mind) which helps with keeping your site up-to-date and secure. If you are interested in knowing more about the plan please contact your Account Manager.