Wiki has the first shot at 9:35 and the last one at 9:46-49 — 11 to 14 minutes. Police were dispatched to the school at 9:35.
http://en.wikipedia.org/wiki/Sandy_Hook_Elementary_School_shooting
News copter video shows police swarming the building at 9:40.
I’m not sure who to believe on the timeline, or whether the police immediately entered the school or staged outside for several minutes, but here is an alternate view of the timeline from what you have, that shows about 10 - 11 minutes from first call to engage from PD:
9:30:00 AM: Classes begin at Sandy Hook Elementary School in Newtown, Connecticut.
9:35:53 AM: First known dispatch call - 911 Dispatch: “Sandy Hook School, Caller’s indicated she thinks someone is shooting in the building.”
9:36:15 AM: Shooting continues - Dispatch: “The individual I have on the phone is continuing to hear what he believes to be gun shots.”
9:38:10 AM: Pause in shooting - “The shooting appears to have stopped. The school is in lock down.”
9:38:50 AM: Police call for SWAT team - “We’ll stage up the SWAT and go from there.”
9:40:30 AM: Reports of shots fired - “Shooter’s apparently still shooting in office area. Dickerson Drive.”
9:40:55 AM: Connecticut State Police receive call and begin sending emergency units from western part of the state - “Troop 8 personnel, take Exit 10, left on 34, turn on Riverside Drive. Make sure you have your vests on.”
9:43:45 AM: Female confirmed shot - Dispatch: “We have one female in Room 1 who has gunshot wound to the foot.”
~9:45 AM: First responders/SWAT team arrive - begin an “active shooter search, checking every door, crack and crevice.”
9:46:20 AM: Another report of person shot - Dispatch: “We’ve got an injured person in Room 9 with numerous gunshot wounds.”
9:49:05 AM: Shooting finally stops approximately around 9:46 AM - “Negative on description. Shots were fired about three minutes ago.”
9:53:25 AM: Suspect reported down - “Newtown’s reporting one suspect down. The Building has now been cleared.”
9:55:25 AM: Weapons report - “Be advised, we have multiple weapons. One rifle and a shotgun.”
9:57:25 AM: Police notice to officers responding to call in plain clothes - “Any plain clothes responding, make sure you have you raid gear on, your raid gear on.”
10:00:15 AM: Police complete search for any other potential suspects - “Ask the custodian, get a team up on the roof and clear the roof.”