They already spider the actual destination page after redirect, so they know where it ends up...Though I'm not sure why they wouldn't just follow the redirect and check for where it ends up.
It does get caught within a few days though usually.
As for the timeframe to catch fake URL violations, I had an ad up for 4 months with a violation after one of my domains expired before they noticed (changed the redirect on my tracking link, forgot to update the display URL).