The balance of prepared formula. left in bottle after baby has nursed  from the bottle is considered OK for one hour.  I went to the Mayo Clinic web site to double check length of time prepared formula can be mixed and kept in the frig.  They recommend using boiled and cooled water to mix and that it is safe to keep in the coldest part of the frig for 24-48 hours.  There are pretty clear directions with all formula on preparation.
 
In 1960 I mixed formula supply for 24 hours with tap well water (no boiling) as that was considered OK then.  My baby wasn't ever sick from her formula.
 
However, today all of this info on formula is for countries where there is a safe water supply and refrigeration  (depends on electricity) availability. This is what makes formula so dangerous in developing countries.  I saw for myself how water is obtained in villages in Uganda and it wasn't a very safe looking process.  Boiling it is time consuming, uses fuel over open  fires.  So it isn't just the expense of formula, the preparation is expensive too.
 
Pat in SNJ

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