Chris, a "sharp" blade means you have thin metal on the leading edge. When it hits something hard like a small tree or sapling, that edge can be nicked, notched, or bent. So I believe it's customary for brush hog blades to have an edge that is approximately 1/8" thick. It shatters and tears brush without damage to the blade. Now if you're only mowing grass and weeds (soft material), then I see no reason not to have a sharp edge on the blades; similar to a lawn mower.
<font color=blue>How do I sharpen the blade and when is it time to get a new one?</font color=blue>
I have removed blades and sharpened them on a bench grinder, and I've also sharpened blades without removing them by using a 4" angle grinder. I suppose it could be done with a hand file, but not by this old man./w3tcompact/icons/laugh.gif That would just take too much time and effort. As for replacement, if it's bent, cracked, bolt holes wallowed out - in other words just inspect them for damage and replace them if they are. Otherwise, I don't know how much wear you want before replacing them. I replaced mine when the "sharpened" section of the blade got narrower than the rest of the blade. When you sharpen them, try to take the same amount off each blade so they stay balanced.