Most phased maintenance programs have a minor inspection due every twenty-five flight hours and a major inspection every 150. The military 150-hour is somewhat similar to the civilian annual inspection. As an example:

If you have a civilian helicopter in the U.S. that you use to haul passengers under Part 91 of the Federal Aviation Regulations (FARs), you need an annual and the recommendations of the manufacturer.

If you are flying under FAR Part 135, the maintenance schedule is quite a bit more extensive.

Of course this varies from country to country. You will need to check the Federal Aviation Administration FAR for each case.

All components of an helicopter have a TBO ( Time Before Overhaul ) or an hour requirement for replacement and the aircraft structure also has time related inspections.

As an example here are overhaul charts from the 23-1 maintenance manual of the AH-1F Cobra :