How many service hours are required for Eagle Scout?

There is no set requirement for the number of service hours to earn Eagle Scout. You are probably referring to the service project component of the rank. One of the requirements for Eagle Scout is to "plan, develop, and give leadership to others in a service project". The project needs to benefit an organization other than the Boy Scouts.

These projects can be fairly simple or quite elaborate. The often overlooked key is leadership. The Scout must involve others in his project. Projects must be approved by an Eagle Board of Review and this group will determine if the project is substantial enough to qualify.

The project must be a significant contribution to the community, and a significant challenge to the Scout. There is no set minimum, and that is explicitly stated in the Boy Scout literature. Statistics would suggest that the average project involves about 200 person hours including planning and carrying out the project, but there have been projects well over 1000 hours, and some as little as 100 hours.
The last eagle scout in our troop needed 96 hours to the best of my knowledge.