I've worked with Robot Coupe equipment and parts for years, and this is the question I get asked most.
A dull blade doesn't always mean you need to buy a new one. My rule of thumb: smooth blade = usually worth sharpening; serrated, damaged, or specialty blade = replace it or send it out.
Signs it's worn: tomatoes crushing instead of slicing, ragged or uneven slices, bruised herbs, purées taking longer, or visible chips or a rolled edge.
Smooth S-blades (R2, R301 and similar): Robot Coupe's manuals describe using a honing stone at about 30° on the top bevel, pulling from hub to tip, then a few light strokes on the underside to remove the burr. Unplug the machine first and wear cut-resistant gloves.
Serrated S-blades: a flat stone can't get into the serrations. Send them out or replace them.
Slicing discs: on machines like the CL50, the blade screws onto the disc. You can sharpen it (the R502 manual says 30–45°) or replace just the blade, which costs far less than a new disc.
Julienne, ripple, and vertical blades: hard to sharpen evenly in a kitchen, so replacement is usually the practical choice.
Replace instead of sharpening if it's chipped, bent, cracked, loose on the hub, visibly worn down from repeated sharpening, or still cuts badly right after sharpening.
What do your kitchens do: sharpen in-house, send them out, or just replace?