To save yourself money buying new batteries and the grief not being able to start your car � buy a cheap multimeter (having both DC Volts & DC Current measuring capability). You should be able to buy such a meter for less than �10.
Having purchased the meter, with the engine running, measure the battery voltage � it should read just under 14 Volts. Next, disconnect the battery negative terminal and with the meter on DC current, connect the meter between the battery negative terminal and the vehicle chassis. The meter will be reading the battery drain, this should be less than 20mA (0.02A) � ensure that everything is switched off including courtesy lights. If it is significantly greater than this � identify the circuit/component causing the battery drain by pulling the fuses one at a time and disconnecting components.
In discussing a similar situation with a colleague, his wife�s car battery was draining within 4 days. After much investigation, he found that if the vehicles alarm was not activated, after around 1 minute a relay coil was permanently energised � it was this that was draining the battery. The solution was to ensure that the vehicle was alarmed whenever left for long periods.