Saving milk or formula. When you use less than a full can of evaporated milk, concentrated liquid formula, or ready to use formula, you can save what is left for the next day. Leave it in the can, cover the top, and keep it in the refrigerator. Use it all up the next day, or discard what's left. Never keep an opened can more than 24 hours.
The same applies if you make a quart jar of formula, or fill all the nursing bottles from one batch of formula. Keep in refrigerator, and use it all the next day, or discard what's left. Never keep bottled formula more than 24 hours.
How long after a bottle has been taken out of the refrigerator can you still use it? During a time when a bottle is at drinking temperature or room temperature or pleasant outdoor temperature, any bacteria that may have gotten into the formula will be able to multiply rapidly. That is why it is unwise to give a baby a bottle that has been sitting around the house or carriage or car for several hours, whether it's a full bottle or one that has been partly consumed.
If you will need to feed the baby a couple of hours after leaving home, put the bottle, as soon as you take it out of the refrigerator, into an insulated bag designed to keep things cold.
If you have a young baby who sometimes goes to sleep after a half a bottle and then wakes up in a couple of hours for the rest, you can simply put the half finished bottle back into the refrigerator. I wouldn't use such a bottle more than twice.
If you cannot keep the formula cold. If you ever get into a situation where you can keep the baby bottles cold until feeding time -- for instance, if you're refrigerator stops working or your electricity goes off -- you will be safe if you heat each bottle before giving it. Unscrew the rings partly, put the bottle in a pan of hot water, bring it to a boil, boil it 25 minutes, then cool it down to body temperature before giving it. Discard all formula that's been out of refrigeration after 12 hours.
If you find this happening to you often, the easiest solution is to use the commercial powder formula, mixing it with water before each use -- 1 bottle at a time. If you have to sterilize, keeping a bottle of distilled water and some disposable bottles on hand, along with the powder formula, can be helpful.