What is the least amount a woman can feed without her milk drying up completely?

How many times must you feed to ensure that your milk doesn't dry up? I have my three month old on formula during the day because that is her most active time and I just don't produce enough for her growing needs but I still want to feed at night when she is settling down. Will I just keep producing even if I am not feeding during the day? Or will I just eventually dry up completely? I don't know how this works. I understand that it is a supply and demand thing but what if the demand is really limited, like only the night feedings (right before bed and sometimes during the night but I find that she wakes too often at night when I only feed so I sometimes get up and make her a bottle so she will sleep more than just an hour at a time). So, back to my question. What is the least amount you can feed and still avoid drying up completely?