I definitely think 3 years is a workable time frame
0 - 3 months      First rumblings from the feral supporters, game plan is crap theme, this bloke doesn't know what he is doing....
6      months      Media latches on and joins the "tigers are crap" theme 
                       Growing army of the ferals gets louder
1      year          Rumours of board coups start to surface
                       Coterie groups start becoming more vocal in their "bring back the good old days"
1.5   years         Coach under pessure from media and supporters to guarentee date when we will play finals
2      years         Board backs coach 100%
2.2   years         Coach sacked caretaker Wayne Campbell appointed
2.4   years         Calls from media, coterie groups, supporters to appoint Sheedy
3     years          New coach appointed
Yep 3 years should be enough to fit all that in.