When going out I take the bottles of water and the dispenser with the formula in it, and make it up as needed.

But at home I make it up about 12 hours in advance. I've been told it's safe to use up until 24 hours, so I do 12 to be sure it's okay. It works well for us because I BF as well, so our usual routine is - boil kettle, BF, put bottle in water, change nappy, then bottle is ready to give!