By the end of the first movie, Marty is just one week older. He entered 1955 on November 5th at 6:00 A.M., and then left November 12th at 10:04 P.M., which totals one week, 16 hours, four minutes. Marty re-enters the timeline 10 minutes earlier, thus making him one week, 16 hours, 14 minutes older than he was on October 26th, 1985, at 1:24 A.M. when he left. At the start of Back to the Future Part II, Marty, Jennifer, and Doc travel to October 21st, 2015, spending less than a day there. Then they go back to the alternate 1985 for just a few hours. We know that the two events take about a day for Marty and Doc from the dialog upon their arrival in 1955, "This is heavy, Doc. It's like I was just here yesterday." "You were here yesterday, Marty. You were!" Marty spends all of November 12th getting the book back. So, at the end of that movie, Marty has aged two more days. In Back to the Future Part III, Marty and Doc spend November 13th finding and fixing up the DeLorean. Then on November 14th, Marty travels back to September 2nd, 1885. Marty is there with Doc until the morning of September 7th. Adding up the time spent in all three movies and approximating a bit, Marty spent a total of about 16 days outside his own timeline.