420 sel a/c

I've had this 1988 420 sel for about a month a think it's a great car. As the weather heats up, the a/C just doesn't seem to be cool enough. I notice some oil leakage around the the flexavble line that leads to the evaporator when I first bought the car that seems to have stopped. How often should the A?C on this car need to be recharged with coollant or is it likely just leaking?