Fly early. The first departures of the morning, usually from around 7am, give you the calmest air, the clearest visibility and the lowest chance of a weather cancellation. By early afternoon the trade winds have built and cloud has piled up against the windward slopes of both mountains, which is where the waterfalls are.
Book the flight for early in your trip, not the last day. Cancellations for weather are routine, particularly on the East Maui routes, and an operator will usually offer you another slot. That only helps if you still have days left.
Allow about two and a half hours door to door for a 45-minute flight. Check-in is typically an hour before departure because every passenger is weighed and the aircraft is loaded to a balance plan, and the Kahului Heliport is not somewhere you can arrive five minutes late. Add the drive: about ten minutes from Kahului, half an hour from Kīhei, close to an hour from Kāʻanapali or Wailea in morning traffic.
Season matters less than the week's weather. Winter brings more rain, which means more waterfalls and more cancellations. Summer is drier and steadier, and the West Maui falls can be visibly thinner. Neither is a reason to choose a month.
One scheduling conflict worth flagging: an air tour is not affected by diving the way the Haleakalā summit road is, because the cabin stays at low altitude, but check with your dive operator if you are flying the same day.