I sharpen mine once or twice a week. I have a small (6") chipper, so sharp knives are critical to getting the most out of the little unit.
I sharpen like Stumper says, a slight curvature on the bevel side alongside the edge. The pic here shows the grinder wheel on the other side, which I do on the final pass, just to knock the burr off.
This is an old picture. These days I use a grinding wheel that is worn down small, past the half way point of it's life. The smaller diameter wheel allows a little better control. You need a pretty steady hand because a slip can screw up your sharpening job.
I wear welding gloves and one of those clear, plastic face shields and ear plugs. It can be dangerous, don't take it lightly. Protect yourself.
I keep three sets of knives and get about six months per set before I feel the performance diminish, even after a razor-sharp sharpening (increased gap between the knife and anvil, I suppose). Then goes on a new set. After another six months or so, the third set goes on and the previous two sets goes in for professional sharpening. At this point I have three fresh sets and in another six months, off comes set #3 and on goes one of the freshly sharpened set. At this point the anvil gets adjusted and the whole cycle repeats.
I've never taken them off to sharpen them, but that's because if I take them off, there's a fresh set to go on. I think it would be easier to sharpen them off the disc, but I'm sorta lazy that way, sharpening them while on is quick, under 5 minutes for the three knives, and I always get a good result.