In the movie Project Almanac there is plenty of time travel, and if I had to guess, based on the montage and some of the comments, the in-movie linear time was several months. However, largely because of all the time travel, it is hard to be sure.
In terms of calendar time, how much time passes in the movie from the opening scene (David making the video of the drone for MIT) until right before David makes the final jump?
Near the beginning of the movie, we know it's February 18, 2014.
Chris: It is February 18th, 2014. My brother has officially lost his mind. He genuinely believes he is building a time machine.
Towards the end of the movie, we find out that it's April 23, 2014.
David: It's April 23rd, 2014. This is to keep a record. I know exactly what to do to fix this. Justin broke his leg at Walker's party. I'll fix that. Everything else will fall into place.
So only about two months elapse from the time they start building the time machine, to when things go off the rails.
From there it's impossible to know how much time passes from David's perspective, because we don't know how many times he goes back, or how long he stays each time. That's left purposely vague.
Jessie: How many times have you done this before? David, how many times have you done this before?
Kevin Workman's answer is correct, except for one thing: the movie shows David going back alone three times before the final jump:
We don't know how long he was in the past for with Lollapalooza and the third jump, but he went back alone three times for at least five minutes before the 2004 jump. See also this timeline.
