John feeds the wild birds in his garden with bird seed placed in three feeders. Two of these containers have the same capacity, each holding 300 grams of seed, whilst the third feeder holds 900 grams of seed. His seed storage bin contains 18 kilograms of seed. Because more birds can visit the large container, it takes the same amount of time to become empty as the smaller ones, and all three containers have to be refilled every second day. How many days will he be able to feed the birds for?