If this is still an R12, and I hope it is, you simply monitor pressures and charge until the sight glass is clear. As the Cap'n said, there is no simple answer to this question. The readings vary from car to car, plus they will vary determined by humidity and ambient temperature.
If the system has been changed to r134, the sight glass is not your key any more. It will require some experimentation to find the correct charge. You will need to ENSURE that pressures are stabilized, otherwise it is easy to overcharge which will ultimately cause damage to the system.
Hope this helps,