From my Land Pride Manual:
Both blades should be sharpened at the same angle
as the original cutting edge and must be replaced or
re-ground at the same time to maintain proper
balance. The following precautions should be taken
when sharpening blades:
a. Do not remove more material than necessary.
b. Do not heat and pound out a cutting edge.
c. Do not grind blades to a razor edge. Leave a blunt
cutting edge approximately 1/16” thick.
d. Always grind cutting edge so end of blade
remains square to cutting edge and not rounded.
e. Do not sharpen back side of blade.
f. Both blades should weigh the same after
sharpening with not more than 1 1/2 oz.
difference.
There isn't much information on when to replace:
"Inspect cutting blades. Make certain they are
properly installed and are in good working condition.
Replace any blade that is damaged, worn, bent, or
excessively nicked. Small nicks can be ground out
when sharpened."
I tend towards sshaw and replace when worn to the point where I can't get a good square end though I have seen a lot of cutters with much worse blades still in use.