The heat kicks in after just a few minutes. We've had a mild winter so far, so those in the NE can probably give you more specifics. However, when the Prius is shut down, the anti-freeze is pumped into an insulated container, and will stay hot for at least several days. When the Prius is started again, the anti-freeze is pumped back to the engine, so there's no cold engine starts. The heat doesn't start flowing to the passenger compartment right away, but I think that as soon as the ICE runs for a few minutes, the heat starts flowing.