Schweinsteiger won't need any time to gel, that's why it's such a clever signing. He's well versed in playing Van Gaal's system and can slot straight into the injured Carrick's place without a problem. Schneiderlin will take anywhere from a few weeks to a few weeks to learn the system, Darmian won't take that long because all he needs to do is bomb up and down the pitch, overlapping with whoever is playing the 'false right winger' position at the time. Memphis will also have no problems gelling into the side because he's also well versed in Van Gaal's system from his time at Ajax and the Netherlands national side.
If we sign Nico Gaitan he's going to take time to gel in.
But any new players that need to gel in and learn Van Gaal's system should do so by November at the latest, the reason why the side didn't get on board around the same time last season was because Van Gaal was only able to field the same side two or three times during the season lol. Mata and Herrera were the last two players to understand things and we all saw a taste of things to come against Spurs, Liverpool and City until Carrick got injured again. Van Gaal's system is dependant on the defensive midfielder orchestrating the movement of the players either side of him and either side of the central striker. One minute the side will look like this:
De Gea
Valencia Smalling Jones Shaw
Herrera Carrick Young
Mata Roonster Fellaini
The next minute they'll look like this:
De Gea
Valencia Smalling Jones Shaw
Mata Carrick Young
Herrera Roonster Fellaini
And then this:
De Gea
Valencia Smalling Jones Shaw
Herrera Carrick Fellaini
Young Roonster Mata
Or any other combinations of those four players' positions. It's this constant switching and movement that completely destroyed Spurs, Liverpool and City and now we have one (soon to be two when Schneiderlin learns what to do as the number 6) replacement for Carrick United will continue playing this fast paced, fluid, unpredictable and attacking football that we caught a glimpse of during those three matches last season.
I think a lot of people are going to be surprised by United once the season starts.