how much formula did you give at 3 weeks?

how do you know when your baby is ready for an increase in their formula? my baby is 3 weeks and has gone from 90ml at 2 weeks old to 120ml for a week now. she polishes off the entire 120ml and doesn't seem satisfied. sometimes she only takes 90ml but that appears to be in the early hours around 4am. is it time to up the amount? and if so to what? health visitor and midwives not very helpful.