Current drain for the alarm is milliamps and as you have already identified that you need a new battery replace it ASAP as once they start collapsing there is no way back and the drain time will decrease rapidly. Once you have the new battery installed, give it a good charge by taking the car for a run and then use the central locking and alarm normally, using the car just at weekends on a new battery will be fine although a battery conditioner won't do any harm either.