How many days do you have to wait so your Pokemon can hatch in the day care in soulsilver?

It depends on what the old man says about how they like each other.

Every 256 steps they have a % chance to make an egg.

If the old man says

The two don't seem to like each other, it is a 20% chance.

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."

The chance of getting an Egg is 69.3% (has a value of 70). The old man in the Day Care will say: "The two seem to get along very well."

The Pokémon are incompatible and therefore will not make an Egg. The old man in the Day Care will say: "The two prefer to play with other Pokémon than each other."