If your vehicle is showing a/c takes a long time to cool down, here are the most likely causes and the parts you'll need to fix each one.
A slow leak reduces the amount of refrigerant available to absorb heat, causing the system to work much longer to cool the cabin. Leaks commonly occur at O-rings, service valves, or the condenser.
A severely dirty cabin air filter restricts airflow over the evaporator. Even if the A/C is producing cold air, it cannot push enough of it into the cabin to cool the vehicle quickly.
Adding too much 'DIY' refrigerant or failing to properly vacuum the system before charging causes excessively high pressures and poor phase change, drastically reducing cooling efficiency.
If the condenser is clogged with bugs and road debris, or if the cooling fans aren't running at full speed, the system cannot shed heat effectively. This is especially noticeable when idling or driving at low speeds.
If the HVAC blend door doesn't close fully against the heater core, hot air mixes with the cold A/C air. This creates lukewarm vent temperatures and significantly increases cool-down time.
Internal wear in the compressor reduces its ability to pump refrigerant efficiently. Alternatively, a worn compressor clutch may slip when hot, causing intermittent or weak cooling.
Debris from internal compressor wear or moisture in the system can clog the expansion valve or orifice tube, restricting the flow of refrigerant into the evaporator.