The title pretty much says it all. From the start of Back to the Future, how much time passed for Marty by the end of Back to the Future III?
I would say ~19 days as listed out below:
BTTF 1 Total ~8.5 days
Marty goes to the Doc's house in the morning. He blows out the windows. Marty meets his friend, scientist Dr. Emmett "Doc" Brown, late that night in the parking lot of a deserted shopping mall,
Doc enters the date November 5, 1955, the date he came up with the concept of the flux capacitor. Marty goes back in time to 1955. Marty shows Doc a flyer he received in 1985, which states that lightning will strike the courthouse clock tower the following Saturday November 12 at 10:04 pm. That night Marty goes back to 1985 after ~1 week in 1955.
~ 7 days
He gets up the next morning and leaves for the future.
~ .5 days
BTTF 2 Total ~3 days
On 26 October 1985, Dr. Emmett Brown arrives in his DeLorean time machine and persuades Marty McFly and his girlfriend, Jennifer Parker, to come with him to the future. They arrive on 21 October 2015,Biff uses the DeLorean to travel to 1955, where he gives his teenage self the almanac, before returning to 2015. Marty, Doc, and an unconscious Jennifer return to 1985 (that night), unaware of Biff's actions.
~ 1 day
Marty and Doc arrive at night. They find a completely different Hill Valley. They try to figure out what happened. Marty confronts Biff. .... Doc arrives and incapacitates Biff, allowing him and Marty to flee to 1955.
~ 1 day
Marty secretly follows the 1955 Biff and witnesses him receive the almanac from his older self (November 12). That night is the concert. After a struggle, Marty takes the almanac from Biff, who crashes his car into a manure truck as Doc and Marty fly away in the DeLorean. With Doc hovering above in the DeLorean as a storm approaches, Marty burns the almanac on the ground and undoes Biff's damage to history. However, the time machine is struck by lightning and disappears. A courier from Western Union arrives minutes later and hands Marty a 70-year-old letter from Doc, who was sent back in time to 1885 by the lightning strike and is now trapped. Marty races back into town and finds the 1955 Doc who, seconds earlier, helped the original Marty from the first time-travel incident return to 1985. Doc is shocked by his friend's sudden reappearance and faints.
~ 1 day
BTTF 3 Total ~7.5 days
On November 12, 1955, Marty McFly discovers that his friend, Dr. Emmett "Doc" Brown, has become trapped in 1885. Marty, with the 1955 Doc, uses the information in Doc's 1885 letter to locate and repair the DeLorean. He spots a tombstone with Doc's name, dated six days after the letter. Learning that Doc was killed by Biff Tannen's great-grandfather, Buford "Mad Dog" Tannen, Marty travels back to 1885 to save Doc.
~ 1 day
He arrives on September 2, 1885, Marty meets Biff "Mad Dog" Tanner and Doc. Doc saves Clara, etc. and goes to party Buford goads Marty into a showdown in two days' time. In the photograph of Doc's tombstone, Doc's name has disappeared, but the date on the tombstone remains unchanged. Doc warns Marty that he, not Doc, might be killed by Buford. The night before their departure, Marty and Doc place the DeLorean onto the rail spur. Events occur..... They coast away from the train as the DeLorean disappears through time, while the locomotive falls off the unfinished bridge.
~ 6 days.
Marty arrives on October 27, 1985. He and Jennifer go driving. Forgoes racing. Doesn't crash, meets Doc.
~ .5 days.
Total 19 days give or take.