Sweet milky liquids are an ideal growing medium for bacteria. Current thinking is to make fresh for each feed.
From the SMA website:
With my last baby I used to make up 6 feeds at a time. What has changed?
In line with the latest guidelines from the Department of Health, (November 2005), we recommend preparing each feed, in individual bottles, as required.
It is important to follow good hygiene practices when making up infant formula and you should always prepare the milk in accordance with the instructions on the specific pack of infant formula.
Once I have made up a feed, how long have I got before I have to use it?
A freshly made feed can be kept at room temperature for a maximum of 2 hours however, if your baby has started drinking from the bottle then it should be discarded after 1 hour.