That is normal operation. If the air temp is fine, don't worry about it. The cycle time will vary depending on air temp and will act differently if you're operating the A/C on fresh air or recirc mode. The thermister resistance signals the control panel to control the compressor clutch relay. The cycle will be longer if you're pulling in outside air; it'll be shorter if you're recirculating the inside air. Be sure to keep the air filter clean, blow out the condenser from the backside, and if you're in a dusty environment, once/ year it doesn't hurt to remove the dash and clean the evaporator with coil cleaner and rinse with water.