There are moving parts to these questions. Thankfully for us, most of those parts and the questions that come with them were answered by a man named Hooghoudt in 1940.
In general, we know that optimal root growth and plant health is typically achieved around 36". In today's farm market, spacing is typically set at 40' but can be as low 15' and up to 100'.
Check out this Drain Spacing Calculator that considers 1845 different soil types.