The standard is 24 to 32 oz. in a 24 hour period. My son is almost 10 months and he used to be drinking 26 oz. a day plus baby food about 2 months ago and suddenly he just won't take that much anymore and starting taking 16 oz. a day if I am lucky. He won't take his formula during the day from a bottle so I have to put it in his baby food so he will get some that way. He also won't take formula from a sippy cup though we are still trying. The only time he will take formula from a bottle is at night when he is half asleep. I talked to my doc at my son's 9 month checkup and he wasn't too concerned because my son's weight was good (he's always been in the 50th percentile for weight).
I think starting at around 9 months babies might not eat as much sometimes because they are not growing as fast as they were. My son also went through periods when he doesn't drink much formula and after a couple days or a week he would drink his normal amount. That was when he was 4 months.
My mom in law also said it could be because by 9 months babies might get tired of the taste of formula especially if you have introduced solids.