Pokemon all have their own rates at which they breed.
If you have a pair that like each other, it goes a lot faster. If your pair hate each other, then they may take months to breed.Try to find a good pair. Pokemon that are the same species and around the same level have a higher chance of breeding.
Legendary Pokemon don't breed. It's not possible, and we don't know why.
These pokemon CAN breed with Ditto, even though they are legendary.
- Manaphy is able to breed with Ditto to get Phione.