Growing up times for all animals is in the Inventory Guide after the A-Z entries. It's the last post on page 3.
https://forums.storm8.com/showthread...=1#post1025446
(it's 21 )
The craft stays there because you can craft extra dragons. Burning embers are one of the rarest items in the game, however if you go on the Desert Sands adventure lots and lots and lots (and then some more), you might be lucky enough to get an ember. Dragon eggs (and Manticore eggs) will drop at random from the Forgotten Roost, so it's worth spending 1 energy daily collecting from it. Plus it's an excellent money return!