To be quite honest, I don’t think anyone (aside from OGame’s programmers) really knows. However, the only piece of “evidence” we have to go on is the fact that the calculators on O-Calc (http://o-calc.com) preclude you from setting a building or research level above 99. Therefore, until we see conclusive proof otherwise, this is what I will assume the theoretical maximum levels to be.
Voidhawk’s Note: Furthermore, the amount of resources and time needed to create any building or research above level 35 or 40 is huge. By the time you hit anything above 50, you need to spend months in build times.