Store video time of Trayvon leaving 18:24 till Zimmerman's call to SPD 19:09 would be 45 minutes. But there is a problem in that you are comparing the times on two different clocks and there is nothing to assert that the clocks are synchronized.
There is a document that shows the police had acquired the surveillance video by late march:
Zimmerman-documents.pdf page 70
2012-03-29 (SA) Dale Crosby conducted a recorded, sworn interview with [store clerk]
stated that he did not remember Martin being in the store.
was shown a video of Martim at the counter
stated he did not remember his interaction with Martin.
But this does not cover the actual acquisition of the video or the verification of the camera's time.
If there were a common event measured by both clocks, that would help establish the synchronization. Looking at the video again:
http://axiomamnesia.com/2012/05/18/trayvon-martin-7-11-surveillance-video-night-shooting/
While the cashier is counting the change, trayvon makes moves that could be reaching for the controls on his phone and in the audio is heard what sounds like "hello, hello". TM's left hand is in his pocket viewed from back right camera. (time: 18:23:57)
And also from the prosecutions documents:
Zimmerman-documents.pdf page 36
2012-04-02 T.C. O'Steen Interview with dee-dee in Miami
"… had talked with him on the cell phone several times on the date of the incident." "The Victim left the store and the phone disconnected. She then called back and the Victim was walking and said it was starting to rain."
This seems to establish that there was an event that could be timed by both the store video clock and the clock of trayvon's phone provider. But if we assume that the clocks are synchronized, the times do not line up:
Code:
11:13 Hollywood text
11:15 North Dade text
11:18 North Dade text
13:30 Miami text
14:20 - 14:23 Incoming 3
14:38 - 16:21 Incoming 103
14:45 - 14:47 Incoming 2
15:02 - 15:04 Incoming 2
15:04 - 15:07 Incoming 3
16:08 North Dade text
16:28 - 17:09 North Dade Incoming 41
17:09 - 18:30 North Dade Incoming 81
[B]18:23:57 -- Call disconnected while Trayvon is at checkout
[/B]18:30 - 18:43 North Dade Incoming 13
18:41 - 18:45 Incoming 4
18:45 - 18:50 North Dade Incoming 5
18:46 - 18:48 North Dade Incoming 2
18:49 - 18:53 Incoming 4
18:53 - 18:54 North Dade Incoming 1
18:54 - 18:55 North Dade Incoming 1
18:54 - 19:12 Incoming 18
19:04 - 19:05 Incoming 1
19:09:34 -- George Zimmerman calls the Sanford Police Department (SPD) from his truck
19:11:33 -- Zimmerman tells the police dispatcher that Trayvon Martin is running.
19:12 - 19:16 Incoming 4
19:16:55 -- Gunshot heard on 911 call
So, not only is there no evidence presented that the clocks between the store video and the police dispatcher are synchronized, there is evidence that indicates that they are not. Until these clocks can be corrected, comparing times between them is meaningless.