my ds is 14w4d old and drinks 4-5 oz 5 times a day. That equals 20-25oz a day, and that is the ideal. Some feeds he only drinks 3 oz and that could be after 3.5-4 hours.
As long as he is gaining weight and having plenty of wet nappies, i won't worry too much, he is but 4 weeks old. About 2-3 weeks ago ds used to drink 3 oz with each feed, which was 3 hourly in the day - 5 feeds- and then two in the night. BTW This was also expressed breast milk, not formula, so technically would be less volume with formula, as it takes 'longer' to digest.
I also found that my ds went through a period of drinking less, but then he had a growth spurt and he is feeding much more now again.
Not much advice, only personal experience. HTH
DI