I do mine in the evening for 24 hours. The guidelines state that you must add water above 70oc (if you boil the kettle, don't let it cool for more than 30 mins) to the formula powder to kill any bacteria in it and then cool it quickly. The longer it stays warm, the more the bacteria within grow to dangerous levels. so if you are out and about, the recommendation is to take a pre-prepared cooled bottle out and put it with an ice brick ina thermos bag and use within 4 hours. But if you are using a frozen bottle or a frozenish bottle, they last longer. If you are going out for more than that, use the UHT stuff. Avent do a great thermos bag which is on offer in tesco atm.
These are the rigid guidelines, but woman have been making formula up with cooled boiled water for years without trouble!