I basically always put off class changing gacha characters from 3* to 4* until I need to to unlock a further 5* class change. I figure it's pretty likely I'll pull a dupe at 4* and then I don't have to worry. I think if you don't use Kid to farm resources it's going to take ... something like from a few days to 2 weeks or so, depending how much light/shadow you have in your party, whether you can run the red key dungeons, and whether you have something better to do with either the red or the green keys. Maybe someone who's done it lately can pitch in with a better estimate, especially if you give more details (how much light/shadow, which characters, how much of the game you've completed).