Well I've always been a BIG fan of Eevee. But today I bread a Vaporeon with my Ditto and in only 2 minutes I had 6 Eevee's!! Then I bread my Golbat with Ditto and in 2 minutes I barley got 1 egg?
Nature and ID numbers determinethe speed of breeding, among other factors. Talk to the DayCare man, he will tell you how long it will probably take for them to breed (although he doesn't give you a time.)
