Does your story start at the beginning and run straight through? Do you pick it up late, and double back to tell the tale? Do you go Memento and get completely unconventional?
Think of structure as the movie clock. Conventional stories are told in linear fashion, from 12:01am to midnight. Sure, there can be flashbacks, but these serve to conventionally drive the story forward. An example of this is The Babe. Pick up The Bambino dropped off by daddy at Catholic school. Follow the fat, unpopular boy through school; see him pick up the bat for the first time, his first baseball contract, his time in Boston, then the New York fame, a best-of series of golden moments until his death.
More unconventionally is Tarantino in Pulp Fiction. He uses non-linear structure, agile transpositions in time, advancing story out of order.
How about Citizen Kane? The movie picks up at 11:30pm on the clock, very near the end. Kane’s “Rosebud” death scene leads to the investigation of what the word Rosebud meant, which leads to a linear structure following the life of Kane, the glorious rise and fall. The whole movie, then, is told in linear flashback. The search for the meaning of the word Rosebud doesn’t come full circle until the final image of the sled in the fire. The movie clocks chimes midnight, and we have gone full circle.
The method of starting the movie at or near the end is often used. Two old Pacino movies I like to compare structurally are Carlito’s Way and Serpico. They are identical; opening with Pacino shot and on a gurney, one as a PR drug lord, and another as valiant real-life cop Frank Serpico. Both movies show how the men were shot, the story behind the shootings. This is useful when the movie is a real life story. 
Milk opens the same way. Documentary footage of Milk’s assassination opens the movie. Why? Because it’s common knowledge the man was shot. Everyone paying a ticket knows that. What they didn’t know is the how, the story behind the story. Who was Dan White and how did this terrible event happen? This is what the movie delivers.
Some movies defy conventional structural boundaries. Mulholland Drive and Memento obliterate standard linear storytelling. In the case of Memento, the story of a man with selective memory disease should play out from a tortured, non-linear POV.
When choosing a time frame for your story, keep in mind there are many patterns beside the standard linear progression. Find what works best for you, and write the hell out of it.
