Are there any rules of thumb when working with Objective-C that would help me understand when is the right time to release variables?
NARC! :)
If you invoked a method that contains N ew, A lloc, R etain, or C opy, then you must release or autorelease. Otherwise you don't touch it.
Of course, anything the documentation explicitly says trumps this rule.
The other thing is that when you're dealing with C function the NARC rule still applies, but also gets the Create rule: if the function contains "create", then you're responsible for CFReleasing or freeing the returned data.