The Phys.org article Bound for Mars—countdown to first interplanetary launch from California says:
In the early morning hours of May 5, millions of Californians will have an opportunity to witness a sight they have never seen before - the historic first interplanetary launch from America's West Coast. On board the 189-foot-tall (57.3-meter) United Launch Alliance Atlas V rocket will be NASA's InSight spacecraft, destined for the Elysium Planitia region located in Mars' northern hemisphere. The May 5 launch window for the InSight mission opens at 4:05 am PDT (7:05 EDT, 11:05 UTC) and remains open for two hours. (emphasis added)
The United Launch Alliance two-stage Atlas V 401 launch vehicle will produce 860,200 pounds (3.8 million newtons) of thrust as it climbs away from its launch pad at Vandenberg Air Force Base, near Lompoc, California. During the first 17 seconds of powered flight, the Atlas V will climb vertically above its launch pad. Then it will begin a pitch and yaw maneuver that will place it on a trajectory towards Earth's south pole.
Mach One occurs 1 minute and 18 seconds into the Atlas V's powered flight. At that time the vehicle will be about 30,000 feet (9 kilometers) in altitude and 1 mile (1.75 kilometers) down range. Two minutes and 36 seconds later, the Atlas first stage will shut down at an altitude of about 66 miles (106 kilometers) and 184 miles (296 kilometers) down range. The Centaur second stage (carrying InSight inside a 40-foot-long payload fairing) separates from the now-dead first stage six seconds later. Ten seconds later, the Centaur's engine kicks in with its 22,890 pounds (101,820 newtons) of thrust, which will carry it and InSight into its 115-mile-high (185-kilometer) parking orbit 13 minutes and 16 seconds after launch. This parking orbit will last 59 to 66 minutes, depending on the date and time of the launch. The Centaur will then re-ignite for one last burn at one hour and 19 minutes after launch, placing InSight into a Mars-bound interplanetary trajectory. Spacecraft separation from the Centaur will occur about 93 minutes after liftoff for the first May 5 launch opportunity as the spacecraft is approximately over the Alaska-Yukon region.
US polar orbit Launches are usually from California due to geography. See How does one dogleg from Florida to a sun-synchronous orbit? for more on that.
But high energy interplanetary launches usually take advantage of the ~0.4 km/sec delta-v "kick" from the rotation of the Earth by launching Eastward.
edit: While I'd originally asked why this launch will use a polar parking orbit, it's clear to me now from answers to Why does InSight plan to launch from Vandenberg that the polar orbit just follows from geography, and that follows from launch site availability and scheduling.
The 59 to 66 minutes sounds like a 3/4 turn around the Earth, and then the 2nd burn of the second state will put InSight and its other payloads on a trajectory to Mars.
Question: How long will the 2nd burn of the Centaur 2nd stage last? How far from Earth (roughly) will it be when Insight is deployed? And will the Centaur do any further propulsive maneuvering (either main engine, or thrusters) to ensure that it doesn't follow InSight or the other payloads all the way to Mars?
below: "NASA's InSight to Mars undergoes final preparations at Vandenberg AFB, Calif., ahead of its May 5 launch date." Credit: NASA/JPL-Caltech