The short answer
An AC that runs and runs is either not keeping up or is not being told to stop. The usual causes are a low refrigerant charge, a thermostat in the wrong place or wired wrong, leaking duct and returns, or a unit that is too small for the house.
What you can check yourself
- Check the filter.
- Check the set point: a very low setting on a very hot afternoon can keep a system running.
- Check for supply vents that are closed or blocked.
- Note whether the air at the vents is actually cold.
What gets checked first
- Refrigerant charge
- Thermostat placement and wiring
- Duct leaks and return air
- Whether the unit is sized to the house
Repair or replace?
Depends on the age. On a newer system it is a repair.
When to call now
If the air at the vents is warm, see the guide on warm air.