I asked my midwife about this when i was pregnant and i was told, Once the bottles have been sterelised, and you have filled it with the ammount of boiling water needed dont add the milk but put the bottles in the fridge to cool. Make up the formula in a milk container and only add it to the water when your baby is ready for the bottle. This way its fresh when the baby drinks it..
24 hours I was told some years ago when mine were babies. I used to make 6 at a time and line them up in the fridge (oh what satisfaction to have 6 full bottles done).