There should be a sticker under the hood, probably on the core support that indicates the exact refrigerant quantity for your car. Failing that, there is a lookup table available from one of the vendors that is linked through www.aircondition.com.

Once you have the system tight and the vac pump running, you should evacuate as long as practical. If it is quite warm when you are doing this evacuation, the moisture will boil out more easily, thus taking less time. If you can evacuate overnight without your pump overheating or other woes, that would be great. In a pinch, you shouldn't evacuate a system that has been opened for less than an hour at a minimum.

Good luck,