This depends on your cat's intake and your availability. If you are going to be away for a few weeks, fill the feeder up accordingly. Also, keep in mind the number of pets here.  If you're someone who stays out most of the time, get a device with ample storage of about 7 liters, and fill it. But, if you're going on a short trip for a few days, less storage will work.    Cats having a larger intake will enjoy a feeder with lots of food. However, there is always a chance of their overeating, so keep that in mind.