That time I used the formula that my pediatrician gave to determine the amount I should feed every 3 hours. Till now my son still needs some food every 3 hours.
The formula that my pediatrician gave is as follows (for 24hrs and for babies on an all-milk diet)
Lower limit: 150ml x weight (kg)
Upper limit: 200ml x weight (kg)
Feeding in between the limits are fine. For amount to feed at ecah feed just divide by the number of feeds per day.
I was starting to train my baby to sleep for longer at night after 1 month. His last feed would be at 11pm and first feed at 5am. By the end of 3rd month, his last feed was 8pm and first feed 5am.