I did some investigation about this for a recent update to SequenceBars.
If your target is a member of your party (fellowship or raid), the objects you get from GetTarget() remain good as long as people remain in your party.
If your target isn't a member of your party, then it will be "forgotten" by the API as soon as you click away. The object's GetName() function will return "". You will no longer be able to assign it to an EntityControl (with SetEntity()).
Originally Posted by souru
i want to know if my current target was in my target before
The only way I've found to do this is to save a set of unique identifying info about a mob (name, base max. morale, etc.) and compare that to new targets. Of course this doesn't work in situations with multiple identical mobs.