To book a helicopter ride you should consider doing so ASAP, so up to a couple of weeks in advanced would be the minimum time, with up to a month or two better still.
When booking your flight you should order your seat online as to be presented with any special offers that may be running at the time of booking.
Ordering a helicopter ride over the phone has its advantages; so such as asking questions that would otherwise not be available online.
So make sure you book your flight way in advanced to be guaranteed a booking.