Most clubs now give the manager a short time to make an impact, if they don't they are sacked, it's their fault simple as that. Average managerial job is something like 11 months, is that long enough? If fans fully backed a manager for a long term appointment would a gradual improvement be seen over time as the club and manager develop. After all we never stop learning, so in theory a manager should only get stronger the greater knowledge he develops of the club and players. Also should a board select the coaching team they think is best for the club. Or, as often seems to be the case, should the manager bring his own team in? These often seem to be his mates who follow them everywhere, but in many cases they don't seem to be the best people for their position. CH's backroom team seem to be a good example. Lets not make this about CH, more a general point.
Its a hard one really, I personally would say if the manager has been back with money then he really should have made his mark in 3 transfer windows, in the case our Mick where he hasn't been backed with transfer funds he must be given a minimum of at least 2 summer transfer windows to realistically be given a chance.
Crap answer this, but of course it depends. I think if you are a top club, gunning for trophies, frankly a manager doesn't need a huge amount of time. The squads are invariably large, with high quality players and generally you will have a good transfer kitty at your disposal. To a lesser extent, the same applies to a club "on the rise" - i.e. Swansea before promotion. If you are subscribing to the same footballing mantra as the club already has, it should be a relatively quick process. The really difficult ones are managers who come in to "transform" the club. I think they will invariably need much longer. Villas-Boas, for example, wasn't given enough time (IMO, though I admit he was poor too). Lambert too needs a bit more time (and also luck - Villa's season more than anyone's has been derailed by injuries). I would definitely include stagnating clubs like Ipswich in this category. The ones it is impossible to say are those plummeting down - Fulham needed results fast and Meulensteen didn't provide that, so he really had to go. He wasn't there to "build", he was there to save.
I don't think you can put a time frame on it, I think its more about having the chance to build the team that they ant and to translate their ideas onto the pitch, once its clear that the team belongs to the manager then they can start to be properly judged.