It depends on the OT numbers.

Different species, different ID numbers

The chance of getting an Egg is 49.5% (has a value of 50). The old man in the Day Care will say: "The two seem to get along."

Different species, same ID numbers

The chance of getting an Egg is 19.8% (has a value of 20). The old man in the Day Care will say: "The two don't seem to like each other."

If the Ditto and Samurott have different OT numbers, then the chance of getting an Egg is 49.5%.

If the Ditto and Samurott have same OT numbers, then the chance of getting an Egg is 19.8%.

So you get a 19.8% chance or a 49.5% chance every 256 steps depending on the OT numbers.

Source