I know that updating websites built on CMS is important for security purposes. However, it can be time consuming if there are many websites to update and even more if some CMS updates require plugin updates (and in some cases, a plugin ceases to work after CMS update which potentially requires a time-consuming solution).
So I was thinking that it makes sense to charge the client for regular updates. There's no reason why it should be free.
How do you guys deal with this aspect ?