Mythic is red X yellow, and the hatch time is 12 hours. Unfortunately, the (very) common Firestorm also a 12 hour red/yellow dragon, so be prepared for lots of Firestorm fails.
The are 8 possible outcomes when using only red & yellow in the breeding den (combos like Fire X Air, Firestorm X Scorpion, Sunlight X Air, etc). They are: Mythic, Butterfly, Clockwork, Fireflower, Firemane, Firestorm, Honeybee, and Wooden.
I don't have a Mythic (on level 143) and have been trying to breed one for over a month now. I've gotten every breedable red/yellow dragon multiple times, including several each of the 3 possible super rares. The number of Firestorms I've hatched is absurd. I've tried dozens of combos, including attempts with my diamonds and many, many basic Air X Fires. Mythic is only a rare, don't know why I cannot breed one to save my life. I'm about at my wit's end with this dragon.
I wish you much better luck than I've been having!
Edit: omg, I just checked the Mythic thread on the other forum and I've actually been trying for this dragon since the beginning of October. Ugh.