Over what time period does Once Upon A Time In The West take place?

Ok so I've watched the film again and compiled this timeline. Contains spoilers

Day 1: Harmonica arrives at the station. Meanwhile the McBain family is murdered (as Harmonica later concludes and explains to Wobbles) Some time later Jill arrives at the station, and sets off with Sam to Sweetwater. She crosses paths with both Cheyenne and Harmonica at the roadhouse She finally arrives at the McBain farm to find her new family dead. The funeral is performed and the posse sets out to get Cheyenne. Sam suggests she comes back with him to Flagstone, she opts to spend the night at Sweetwater though. Once alone she starts searching for the money, she suspects must be there. Meanwhile Harmonica beats up Wobbles Jill in her search finds the model buildings. Harmonica plays in the night. (hoping to catch Frank there?)

Day 2: Jill has decides to leave but is held up by Cheyenne, who's shaken off the posse and has smelled money. Morton and Frank discuss what action to take next, now that a Mrs. McBain has turned up. Frank decides to kill the widow : "My weapons might look simple to you, Mr. Morton, but they can still shoot holes big enough for our little problems. Pretty soon the widow McBain won't be a problem no more." Cheyenne takes his leave from Jill. Jill again tries to leave, but is now held up by Harmonica who saves her from the assassination by two of Frank's goons.

Some time passes, off screen Harmonica convinces Jill that Frank is the one who killed her family, and convinces her to trick Wobbles into giving away Frank's location. What follows could very well still be day 2, perhaps a day later.

Jill tells Wobbles she wants to negotiate with Frank in person. Wobbles goes to Mr. Morton's train to warn his bosses. Harmonica tails him. Frank notices the shadow of Harmonica on the train's roof and makes the train leave the station. In the desert they meet up with Frank's gang and Harmonica is caught. - Flashback 1 - Wobbles gets killed (trying to give away Cheyenne) Frank wants to interrogate Harmonica, but Morton insists on his priorities. Frank leaves to take care of the wido personally. Cheyenne springs Harmonica.

Lumber and tools are delivered to build a station and a town around it. Jill realizes what McBain's plan was because of the station sign. She rushes back to find the model station, but is met by Frank who beat her to it.

Some days later?

Mr. Morton and Frank are in the mountains (one of Frank's hideouts?). Apparently Jill is also there (kidnapped?). Morton forbids her murder. Frank's beard has grown considerably. it's safe to assume a few days have passed. Frank is convinced he'll be the new owner Morton will have to make a deal with.

Meanwhile Harmonica and Cheyenne's gang start work on the station.

In Frank's mountain hideout he has sex with Jill, who'll do anything to save herself. They come to a resolution that doesn't need her being killed : a rigged auction.

So between then and the auction in the next scene, at least some days must have passed. Setting up the auction will take some time.

A week later?

The auction starts, Frank's goons subtly browbeat any takers into not bidding. Morton meanwhile buys Frank's men off, hiring them to kill Frank.

The auction concludes, Harmonica wins the bidding, offering Cheyenne's award money as payment. We see one of Frank's goons leaving Morton's train. (Presumably to go and kill Frank) Cheyenne's shipped off to Yuma, but it's clear his men will spring him free again.

Frank tries to negotiate with Harmonica. - Flashback 2 - Subsequently Harmonica saves Frank from his own men. Frank leaves to kill Morton, only to find the remains of a massacre between Cheyenne's gang and Frank's men. Mr. Morton is mortally wounded. Frank sees Morton's death struggle and leaves. Morton dies.

Frank travels to Sweetwater. The tracks have reached Sweetwater (indicating indeed that several days passed since Jill agreed to the auction) Cheyenne arrives and has coffee and attempts a shave. Frank and Harmonica have their showdown. - Flashback 3 now complete - Harmonica and Cheyenne leave to never (?) return. But Cheyenne reveals he was shot by Morton and is in fact dying himself. Harmonica rides off with Cheyenne's body. Jill takes up her new role.

Conclusion

A lot happens in the first two days. Then we apparently skip a day maybe more. Frank's beard being the sole indicator. Setting up the auction, and the advance of the railroad suggests several more days pass before the final day.

I don't believe the unknown skips are much longer. There is a sense of urgency to erecting the town of Sweetwater, and when the railroad reaches Sweetwater, there is indeed a station, but it's far from finished.