If your engine has a data plate from Lycoming on it and your prop is certified for that particular Lycoming model, then you will get 25 hours. Anything else gets you 40 hours. You will have to show the inspector the certification sheet for the prop to prove to him it is a certified combination.
Now, if you pull that Lycoming off your experimental and try to sell it, it cannot go into a certified airplane again unless it is completely remanufactured (not rebuilt) by a certified engine shop. It has to do with the fact that on a certified engine, the parts from the same engine stay with the engine until they get replaced. When it's remanufactured, it's a new engine again and it comes with a new logbook. On an experimental engine, parts from different engines can be mixed and matched as long as they are within tolerances for a rebuilt or a remanufacture. That's why Aero Sport Power engines come with a new data plate that identify it as a "Aero Sport Power O-360" instead of a Lycoming.
However, the Feds do make mistakes. I got only 25 hours on my RV-6 because the inspector failed to notice that the engine was from Bart instead of from Lycoming. My Sensenich metal prop was certified for my O-320 so he only gave me 25.
For a simple, VFR airplane, 25 hours of testing is usually enough. But I almost prefer the regime of a 40 hour test period because you really need it to do flight testing properly on a more complicated airplane, just my $.02.