I have a Shuckle in Crystal, and it's holding a Berry. How long does it take for it to turn into Berry Juice, and how long does it take the Berry Juice to turn into Rare Candy?
A Berry will only turn into Berry Juice for the first Shuckle with a Berry in the party and only if the player has reached Goldenrod City. Once the prerequisites have been met, there's a 1/16 chance after every battle.
Berry (Bulbapedia) Pokegold (GitHub)
It doesn't take too long, if you run up and down the route to goldenrod city it should be done.