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If the temperature is normal when the a/c is first turned on but then eventually warms up then it's probably not a charge issue, unless it's working ok when the outside temperature is cooler and blowing warm air when it's hotter outside.
It wouldn't be a bad idea to at least check the charge. The best and accurate way to do this would be to recover what is in the system and then charge it to the correct amount with a recovery machine. This is the only way to know the correct amount is in the system.
A gauge set will usually get you in the ballpark provided the system is working correctly. If there is anything wrong in the system though it will affect the pressure readings to they can't be used for charging.
Assuming charge is actually ok then you could have a compressor clutch or coil issue, a bad low pressure switch, high pressure switch, evaporator temperature sensor, poor airflow through the condenser due to blockage or a bad fan clutch, orifice tube, or other blockage in the system. You'd first want to see if the compressor is still running when the temperature warms up. If not then you'd need to investigate the electrical side of the system, if it's still running yet blowing warm the you'd need to start with the correct charge and then see what the pressures are doing when it acts up.