Short answer: there is not one universal number, but for a normal urban bench you should think in the range of 6 to 8 feet, or about 1.8 to 2.4 meters, before a middle leg becomes the easy solution. A well-designed metal bench can often span 10 to 16 feet, roughly 3 to 4.8 meters, without a center support. Beyond that, you are not really buying standard street furniture anymore; you are designing a small beam structure.
The real limit depends on three things: material, cross-section, and how much bounce people will tolerate.
Wood is the strictest material. Most wooden slat benches use boards that are only 1.5 to 2 inches thick. Once you push a wood seat much past 6 feet, it starts to feel springy. By 8 feet you usually need thicker boards, a hidden steel stiffener underneath, or a middle leg. With a beefy hardwood section, 8 feet is achievable, but 10 feet is asking a lot of a standard wooden bench.
Steel and aluminum change the picture. A steel tube or channel frame can hide a lot of stiffness inside a slim profile. Many off-the-shelf metal benches are comfortable at 3 to 4 meters without center legs. Aluminum extrusions can handle similar spans if the profile is deep enough, but they may need internal ribs or reinforcement to avoid a hollow, drum-like feel.
The engineering reason is usually deflection, not just strength. A bench tends to fail comfort before it fails structurally. If the seating surface deflects too much under people, it feels unstable even if it will not break. That is why span tables often limit deflection to around L/240 or about 10 mm for furniture. The formula punishes span length hard because deflection increases with the fourth power of span. If you double the span and keep the same section, deflection can increase by roughly sixteen times. So going from 6 feet to 12 feet is not twice as hard; it is much harder.
Installation details also matter. A bench with two end supports acts like a simple beam. If the ends are bolted down rigidly or the frame is continuous over the supports, you can gain some span. If the bench is cantilevered from a wall or a central spine, the rules change completely, and the unsupported length is usually shorter.
For practical urban projects, here is the rule of thumb I use: wood slat benches without a middle leg are comfortable up to about 6 to 8 feet. Metal benches with a decent frame can go 10 to 12 feet. Higher-end engineered benches with deep steel or aluminum sections can reach 14 to 16 feet. Custom structural designs can exceed 20 feet, but at that point the cost, weight, and foundations usually make a middle leg or central support the smarter choice.
So the longest practical span for something you would still call an urban bench is around 4.5 to 5 meters, or roughly 15 to 16 feet. You can go longer, but you are fighting physics, material cost, and user comfort just to remove one leg.