at first I did it for 10 seconds at a time until I got used to how long it took.
I make up 3-4 bottles right before bed for Colton's night time/early morning feeds. They are 4ounces but since they have been cold I normally have to do them for 40+ seconds. 40 seconds gets them to room temp, but I do 20 seconds, shake, and then 20 seconds, shake. Then I usually do 5-6 more seconds to warm it a bit and help him go back to sleep.
I normally just do him day feeds with slightly warm water instead of microwaving them. It's just easier at night because I dont want him getting into a full out scream while Jamie's sleeping (has to wake early for work) and I'm trying to make his bottle.