DIAGNOSIS Your air conditioning system is undersized, low on refrigerant, or has a airflow blockage. The Nest is calculating the time based on how slowly the temperature is dropping.
FIX Check your air filter. A clogged filter is the most common cause. Replace it if it's dirty. Then, go outside and check the outdoor condenser unit. Ensure the coils are not clogged with leaves or dirt and that the fan is spinning.
IF THAT DIDN'T WORK The system may be low on refrigerant or have a failing component. The Nest is accurate—if it's taking forever to cool, the system isn't producing enough cold air. Listen for the compressor cycling on and off quickly (short cycling), which indicates a refrigerant issue.
HEADS UP This is not a thermostat problem. Continuously running an AC that can't keep up can freeze the evaporator coil and cause compressor failure. Call for AC service.