Board Thread:Dragon City Gameplay Questions & Answers/@comment-5659983-20130803010011/@comment-10090661-20130803012842

It is easier to look at the gems to determine the number of hours. (For time >= 1 hour) It is 1 gem/hour, so 1 gem = 1 hour. No math required. xD

Currently the only Dragon with a breed time of 42 hours is the Pure Dragon, like Simonetti said. =D