If you check your filter lists, you may see that updates are almost always several days out of date. Whether you need to do anything about this depends on which device you're using.

Desktop (Windows, OS X, Linux, whatever)

The update frequency is set by the list maintainers and is stated in the list itself. If you look at EasyList, for example, you will see on line 6 that it expires every four days. AdBlock reads this and understands that it only needs to update that list every four days. This helps EasyList's server not to get overloaded with requests for updates.

Bottom line: Usually you don't need to do anything. If you're suddenly seeing ads or something isn't working right, however, a good first step in troubleshooting is to manually update the filter lists.

iOS (iPhone, iPad)

Apple determines how often the filter lists (AKA "content blockers") are updated. The lists can go weeks between automatic updates.

Bottom line: It's a good idea to manually update the filter lists at least once a week.