impact? it will be a story for someone locally, regionally and some nationally. it could be a big national story if the right guy were to come in.
but as to on field impact, i would think it would take at least 2 years to see significant impact as the current roster learns the new system, and another 2 years past that before the new coach is dealing primarily with players he and his staff recruited.
i think with any coaching change you have a window of about 4-5 years to really see what that coach is all about.
the right guy can come in and do what Saban and Meyer has done, but that requires some very good recuriting early on so you're playing your guys in year 2 instead of year 3, 4 and beyond. getting your guys in your system as soon as possible is key.
for example, Michigan and Rich Rodriguez would be in a considerably better position had Terell Pryor gone to Michigan rather than OSU. Pryor fits RR's system and could have been an immediate impact. now, RR has to go find others to fit the bill, thus holding up some progress.....