Ideally, I’d say between two and four years apart. That way, the parents aren’t overwhelmed with too many babies at once, and the siblings are close enough in age to share interests and experiences.

However, it doesn’t always work out that way. You may get something like the March sisters in Little Women or the Boone sisters (Debby and Company) in real life, with the births a year apart, maybe two. Fertility can happen that way.

Or, you may get a situation like in Family Ties or Blackish, where the parents have teenage and preteen children and, bam, a new baby is on the way. A similar thing happened with my aunt and uncle.

Even if you have children “properly” spaced apart, if you have enough children, the oldest will be much older than the youngest. In that case, the oldest siblings will be like second parents to the youngest siblings, which has its blessings and its drawbacks.