I am planning to get into the concrete pumping business, and would like to understand the life of cylinders and mud cups. How many hundreds or thousands of yards before they typically wear out?

Hard to say. Too many variables. The biggest factors being pump upkeep and conveying material.  Upkeep referring to, ensuring's a central lubrication system is lubricating the piston with oil on each stroke and the section at the most bottom point of the cylinder transition section (NOT THE CYLINDER ITSELF) is a fraction higher (built up via hard surfacing) to protect the cylinder inlet. Lubrication of water in the water box to keep things cool also helps significantly

Cylinders come in all types and forms and rubber plungers too noticeably solids or multi piece. The Muti Piece tend to last a little longer as they are made of more durable material but can wear the cylinders a little faster also.

Conveying material matters just as much. The more "pump friendly' the concrete material the longer everything lasts. Pumps running at higher delivery pressures and speeds will experience more wear than the alternative.

If all is kept to standard, assuming you don't get the misfortune of pumping some rebar through your cylinders, up to about 2000 Pump hours cups should give you 350 Hours of piston life. After 2000 Hrs about 250 and around 6000Hrs the cylinders would more likely need to be replaced. This is however, best case scenario and worth noting each manufacturer has their own manufacturing specs and quality of materials they use.

To be safe, assume as an owner that each cylinder gets replaced after 2000 pump hours pistons every 200-250 and you will get more worth from your machinery.

just curious, why do yu go by pump hours, rather than by how much concrete you pump?