If you're asking what to expect when you're watching it under the hood, you should see it click on and spin the compressor for typically 15 seconds to a minute, then click off. It will stay off for about the same amount of time if the system has a full charge of refrigerant. As the charge leaks out the compressor will run longer to pump the required amount of refrigerant, and it will stay off for a shorter period of time. Rapid cycling, like every one to five seconds is a sign of very low charge. Once the charge gets low enough, the low pressure cut-out switch will prevent the compressor from running because if there's a leak and the low side gets pumped into a vacuum, air could be sucked into the system. Air won't hurt anything but it's the humidity in it that causes trouble. Water and refrigerant R-12 combine to form hydrochloric acid which will eat away metal parts.
Some systems use a variable "wobble plate" in the compressor to adjust the flow of the refrigerant. Those compressors will run continuously when the AC or defroster are turned on.