The SDI-12 protocol limits cable length to 1000 ft, but the cable properties and power requirements of the CS650-L and the CS655-L are such that communication with a control port may work for cable lengths greater than 2000 ft. If multiple sensors are communicating through the same control port, the total length of all of those sensors must not exceed 2000 ft.
In practice, it is less expensive to purchase a new datalogger than to buy a CS650-L or CS655-L with 2000 ft of cable. If the cable is run through conduit, or if a 2000 ft long trench needs to be excavated, then the installation cost becomes more expensive than buying another data acquisition system and sensors with shorter cables.