Just use them until they wear out, then put on new ones. Pay attention to the bolts, they wear out too.
The top, trailing edge of the lift fold and the leading cutting edge slowly erode away.
Unless you are close cutting regular cuts on fine grass (couch, Qld Blue etc) half worn blades are as good as new ones.
If you want to sharpen blades…
draw file them regularly
if using a powered grinder, don't make hot spots which will soften the metal, just use successive, quick wipes along the faced edge.
Sharpening usually causes balance problems.
I normally only sharpen full bar blades and then balance them, I don't bother with cheap rotary swing blades.
edits by DJ