AS3 variable declarations

I was thinking about how to streamline my code the other day and looked through my classes checking for possible memory leaks. I wonder when are variables declared inside of methods clear for garbage collection? or are they not and just hang around?

For instance if I have a class like this:

public class MyClass {

    public function myClass(){}

    private function someMethod():void{
          var a:SomeOtherClass = new SomeOtherClass;


Since the variable was declared without reference into the class how long does it stay in Flash’s memory, or does it stay indefinitely because it cannot be nullified? Thanks for the hlep.