Short answer: There is no single standard value - spacing is calculated per span and suspension type. Common practice: uniform spacing within a span, generally not exceeding 12 m, typically 5-8 droppers per span (more for longer spans).
Why uniformity matters
Dropper spacing sets the support-point density of the contact wire: closer spacing improves elasticity uniformity and reduces contact-force fluctuation; wider spacing increases mid-span elasticity and height variation. Non-uniform spacing creates periodic stiffness steps that accelerate local wear and pin fatigue. Current-carrying droppers add parallel section for heavy-haul, high-current sections.