Ah, one of Zack's favorite quibbles.
Most of the approximations here are actually fairly good. Commonly, a continuous beam will rake over a surface rather than bore in a single spot, so it behaves as most high Rate-of-Fire weapons.
There's actually one good example of a continuous beam weapon: the aether saber-rifle used in melee. Usually, determining the result of such slashes are pretty much eyeballed. Same difference, really.
It seems you already have a good mental picture of how devastating a continuous beam is... so, any damage/5 seconds approximation seems beyond the point.