I am not sure it will even take three years. If they don't win at least 7 next year, both will be on the hotseat immediately. If the season is a disaster, losing record or worse, both will be gone by the end of next season... mainly, because this looks like a crony hire by the AD. He is really sticking his neck out, and if it doesn't look like it will pay off immediately, the fans and boosters will turn on both of them very quickly.
As it is, it's not going to be a popular move. As an ASU fan, I hope it works out, but I have my serious doubts. Edwards was never a successful coach on the NFL level, has no college experience, and frankly, has always sounded like an idiot to me as an analyst. Maybe he is a great guy and a great motivator - I can't say. But he had better be able to get the program winning quickly next year, because there won't be a lot of patience from the fanbase given the circumstances of his hiring.