There are two types of stale. One is what has already been discussed, the type of stale that is in the opinion of the viewer.
And there are some people who have quite literally done everything as both a face and heel in every chacter possible that they indeed stale. I went into depth about how stale Triple H is and you need look no further than WrestleMania to see how stale the man is. If you get ZERO response in the main of the biggest show of the year, you are stale . He has quite literally done everything and has been shoved down our throats for the past eight years through his constant political crusades that fans really don't care due to the severe overexposure. And in this case, the only thing that could "freshen" HHH up is keeping him off TV for awhile or wait for him to tear another quad. Hogan was stale as shit as a face until his turn and subsequent nWo run, but there is nothing that Trips can do that he hasn't done, he is a guy who is quite literally run his persona into the ground.