There is no minimum hours. Your have to build %51 of the project yourself to meet the requirements fo experimental certification. If the total project is 10 hours your have to build 6 hours yourself. Even if the kit manufacture sets up a factory to do assembly in 5 hours that would take you as an individual 3000 hours. Your are correct that people under estimate the effort involved in most kits. However you as the builder are the licensed mechanic and assume the liability as such. The FAA will happily let your plow yourself into your own grave aslong as they are reasonably sure no one else will get hurt in the process. There are some basic flight test you must perform to certify your aircraft, High speed taxi - run the aircraft down the runway without taking off and see if the gear falls off an the engine maintains power. Fly the plane a brief period in ground effect to to see if it is controllable in flight. In is called opening the envelope (go read the "Right Stuff"). All in all the requirements are probably scarily minimal when it come down to it. Nad if you are smart enough to build the plane in the first place probably a process you would see as prudent. And for amazingly small amounts of money private test pilots will risk testing your aircraft. After you personally fly your aircraft within 50 or 100? miles of your base airport for 50 hours you are then allowed to be certified as experimental. Engines are the tough part. You cannot manufacture them yourself and they have to be reliable. Liability is an issue for manufacturers so they are not cheap.
Actually for boats the requirements are quite rigorous and enforced mainly because you can usually pile more people on a boat than a home built aircraft.
