DS3 is three weeks old.
In his first few days he typically fed every two or three hours, but did a weird seven-hour stint the first night. This was because he was coughing up a lot of mucus and swallowing it, so was constantly full 
DS2 was the same (swallowing mucus) but once that had gone was 2-3-hourly. The hospital then had a policy of waking them after three hours, and supplementing (syringe feeding at least 1ml of expressed milk per feed) after five hours.