You never need to recharge it if there is no leaks.

You can tell the freon is low when it a/c is not cold, and you hear the a/c pump goes on and off a lot more frequenily than normal. But you will need to confirm with the gauges, it could be other reasons.

Again do keep in mind, more freon doesn't mean it gets cooler (it actually get less effective). It needs a balance (or difference actually) of high and low pressure to suck the heat out from the cabin.

NEVER NEVER NEVER overcharge the system, you will get more trouble.