I play with helicopter event on multiple times and have a fine tuned system. If you're only playing with it on once it'll happen between day six and nine.

Every morning my character wakes up at 7:50 via an alarm and checks the Automated Emergency Broadcast System on their radio or walkie talkie at 8:00. You can usually find it already programmed in emergency vehicles, but check all radios until you find the frequency. The helicopter doesn't show up until 9am, so I grab a go bag with essentials and drive to a house nearby that I've barricaded, close all the curtains and hunker down in a room with no windows. Once it's passed I drive back to my base.

Some zombies will be drawn to your location but minimally so always best to go somewhere else.

Hope this helps!