Baby bottles should be washed and sanitized everyday.
It's best to have a few different baby bottles available that you can make formula in or put breast milk in so that you can have a fresh clean baby bottle for your baby to feed from.
After the baby has drank the milk from one bottle I would place that baby bottle in the sink or dishwasher and then wash it and use a fresh baby bottle to feed my babies with.
Baby bottles are cheap and can be bought for around $1.00 each so I had around 10 baby bottles available so that I always had clean baby bottles.
I would also sometimes dip the baby bottle nipple into some boiling hot water for a few seconds to help kill bacteria and I would do the same with pacifiers.