If you want to change the variable in the runtime, you'll have to make an object to store it in an attribute (e.g. an ApplicationSettings object with an attribute for how long inactive user accounts are kept). If you make a find or create flow wherever you need the object, you should always only have one, so performance impact and such on your application should be minimal.
You can create a Constant in your module. Constants can be used in many places.
Its really handy and is similar to Global variable in any programming language