Media analysts get this question a lot. I certainly do. How long can you run a TV creative, before it 'wears out' and is no longer effective?
Still, I've never seen a reliable piece of modelling work that showed a creative had worn out and wasn't generating sales any more.
Us analysts don't like this sort of question. It's not answerable with a nice, billable, 6-8 week econometrics project. It's also generally asked in a way that implies there should be a general rule for this sort of thing. If somebody showed reliably that for a few advertisers the creative was definitely starting to wear out after 18months, that would be it. 18 months set in stone and wheeled out time and again in presentations.
If anyone reading is looking for a rule of thumb, I didn't just say it was 18 months!
Lets be honest, it depends on the creative and what you use it for. Pepsi's new Superbowl ad is going to get old quickly (ineffective? maybe, maybe not.)
This monstrosity from America is old almost instantly and can't sink any lower after that. Bet it still works though.