I've been playing Fire Emblem: Mystery of the Emblem and while using Banutu, i noticed after a while he reverts back to a manakete. fighting an enemy seems to extend this but he will still revert after a while. so I don't end up using all of his dragonstone uses, how long after using Banutu's dragonstone do i have before he reverts to a manakete?
According to https://fireemblemwiki.org, Dragonstones last for 5 turns, with the exception of the Earthstone, which will turn Medeus for the remainder of the chapter.
The different Dragonstones available in Mystery of the Emblem:
All of them have 18 uses, adding up to 80 turns of transformation per stone.