I do the hot/boiling water for 5-7min (depending) because the one's baby DCg has are the drop-in-liner bottles and the thin plastic of the "bags" heat up very quickly.

Now when my daugher was on bottles her's were the Born Free ones and it's a thicker plastic that took over 15min to heat up! When a baby is hungry 15min seems like 4hrs! At that point I'd try to time it so it's ready before but 80% of the time I microwaved it...25sec! I'd make sure that the only thing going in was the bottle and shake it up very well.