When I place urban benches, I use 36 inches as the hard minimum between them. That matches standard accessible route clearance and lets one person using a wheelchair, walker, or stroller pass without scraping the seat edges. But minimum does not feel comfortable. If you can, leave 48 to 60 inches. At 48 inches, two adults can pass each other naturally. At 60 inches, you also get a small social buffer for people sitting and people walking.
Think about the full walking corridor, not just bench-to-bench distance. If benches face each other, the clear zone is the space between the front edges. In a neighborhood park, 48 to 60 inches works well. In a busy transit plaza or shopping street, go up to 72 inches or more, because people will stand, carry bags, and move in groups. Also avoid placing benches so close that a seated person's legs or a dog leash block the path. A good trick is to mark the area with chalk or cones and have two people walk past someone sitting, then adjust until nobody has to turn sideways.
So my rule: 36 inches is the code-style minimum; 48 to 60 inches is comfortable; 72 inches or wider is best for high-traffic urban areas.