In terms of timescale, a manager for United should be evaluated as soon as they get the job. The microscope from the club should immediately be on them. It's a high level job and requires someone to come in ready to perform. There should be signs of good football from the very start of the season. In terms of actual style and performance, we should at the very least be seeing that by November. At United, we should be able to control and win games, creating enough chances to make that the likely outcome for each game. That, imo is good football. It doesn't have to be wing play or gegenpress or tika taka. It just needs to put our players in the best position to consistently win games. Our fans act like that is impossible to do. It's not. You don't expect Bayern to keep a manager who puts out football where the team is struggling to create. Real Madrid and Barcelona wouldn't do that either, and neither should we.
The fact of the matter is that as much criticism as United players have received in the last 10 years, our squad has always been a top six squad in the league. We play at the very least 14 teams that we should be better than every season. We then also have cup games and Champions League/ Europa League games we play too. You can't tell me that we should not be expected to play well against those other 14 teams, half of which would be home games. Showing potential in a previous job does not mean you'll be a good manager at United. Just like players, it's the potential that got them the job in the first place. Performing well is what keeps you the job, not past performances in smaller institutions. Even then, on reputation, Mourinho is the only manager that should have been given the benefit of the doubt. Van Gaal, Moyes, Ole and ETH should all have had to earn it. They weren't given chances because they showed potential whilst managing the club, they were given the chance to continue because we weren't bottom the table. These managers only lose their jobs once its impossible for them to actually keep it.
You can quantify a manager's initial success. Feel is emotional, it can be obscured by so many things and when it comes to business with fans and others around the club who have expectations, feel should never be enough. A combination of what you can see ( performances on the pitch, behaviour off the pitch) and results should be what is assessed. I'm not saying you sack a manager without context of injuries or getting familiar with the league ( which are outside factors), but even at the very beginning performances and results should always be addressed. My take is that the managers we've had and the leeway they've gotten is what's responsible for the squad profile. I've seen Mancini manage bloated disruptive squads at City during their transition to a top team. It's part of the job. Being able to communicate and manage personalities effectively. It's not an excuse. Managers at United have been using that for too long. Madrid, Barca, Bayern all have massive squads with big egos. That's part of the test.
The best managers can say they got teams to consistently play good football. Guardiola's City even in his first season played good football. United, even at our very worst under Fergie were always able to create chances. Wenger at Arsenal, despite not always winning, can say he consistently played good football. We aren't Burnley either. This is United, we outspend and have consistently outspent teams over the years. We have the resources to ensure that we play good consistent football. No matter the job, there is always an evaluation and there are standards that need to be met. You argue that the managers prior didn't get the time to complete their projects. My argument is that they didn't deserve the time to do so. The managers actually failed their evaluations and should have been let go way before the time they actually left.
There is a way to evaluate these managers: performances and results.
I beleive it can be. You can't just give someone a project and close your eyes and expect them to succeed because they told you they were going to. We are fans. We can watch a game and assess if we played well or not. You can be able to observe our ability to create chances, our ability to control games, our ability to transition, our ability to defend. This can be seen by simply watching our team consistently, the same way scouts watch players to know if they're good enough to pursue. I don't need Antony to play for me for me to know that he's not going anywhere in three years. I don't see what's hard in that. In a season, we have 45+ games. It's a large sample.