That's an excellent question that gets to the heart of creating comfortable and functional public spaces. As an urban designer, I don't think of it as a one-size-fits-all number, but rather a balance between efficiency, comfort, and social interaction.
A common industry standard for a comfortable, personal seating allowance is about 24 inches (60 centimeters) of linear space per person. This gives someone enough room to sit without feeling cramped, allowing for a natural posture and a small personal bubble. You'll see this spacing in many transit stations and formal plazas.
However, the magic of a great city bench lies in its flexibility. In a bustling park where people want to chat, read, or eat lunch, I might design for a more generous 30 inches (75 cm) per person. This invites relaxation. Conversely, on a busy downtown sidewalk where the primary need is a quick rest, we might design seating with subtle, molded dividers or armrests spaced at 20-inch (50 cm) intervals. This efficiently defines space for more users while politely discouraging lying down.
The bench's depth and backrest are also crucial. A minimum depth of 17-20 inches (43-50 cm) is essential for stability. A backrest, even a slight one, significantly increases perceived comfort and allows people to occupy the space for longer periods comfortably.
Ultimately, the "right" amount of space depends on the bench's purpose. Is it for brief respite or lengthy contemplation? The best urban benches are designed with empathy, offering not just a place to sit, but an invitation to experience the city.