I can't speak at all to preemies (that's probably a more appropriate Q for a pediatrician), but 36oz/day is totally normal and healthy for that age range. Our 3 month old sleeps 11 hrs (undisturbed) and takes around 4 short naps (30-45min) during the day. She eats 5 times during the day, each 6-7oz. Our pediatrician's feed guidance chart actually suggests the "average baby" eat a little more than she is eating currently, but her weight gain is on target with what she's eating now.
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