Originally Posted by Bogeywoman

Put some effort in, man. Your question could be answered by looking it up in wowwiki.

There's a 15 second timeout after the last application of the diminishing returns spell. So you can fear someone to immune, and then go do something else for 15 seconds, and then come back to start it all over again. But that's how the game has worked since 1.4, released 4/8/05.