Not only is overcharging your system potentially dangerous, an overcharged system will not function properly and can seriously damage your A/C compressor or other component parts. The symptoms of an overcharged A/C system are exactly the same as a system that is undercharged: warm air out of the vents.
In order to prevent overcharging your A/C system it is imperative to measure the pressure before beginning to charge it. To measure the pressure in the A/C system, attach a pressure gauge to the low side port. Your car must be running with the A/C system set to maximum cool and the fan in its highest setting. It is important to note that the proper pressure in your A/C system is dependent on the ambient (outside) temperature. To determine the proper pressure for your system consult a pressure chart such as this one, or set the bezel arrow on your pressure gauge to the corresponding ambient temperature and charge to the pressure corresponding to that temperature setting. If the outside temperature is at or below 55°F do not charge your car. Also, never charge your vehicle above 55 Psi.