Great question—the short answer is that most street benches are not fully pre-assembled, but you won't need a few hours per bench either. The majority of commercial outdoor benches ship partially assembled or knock-down. The seat and back sections are often pre-built or only need basic bolt-together work, while the legs, end frames, and armrests may need to be attached with the included hardware.
For tools, plan on a socket wrench or ratchet set, an Allen/hex key, and a rubber mallet to help align parts. A cordless impact driver can make the job faster but is not required. For a standard bench with straightforward hardware, two people can usually finish assembly in about 20–45 minutes. You would only get into “a few hours per bench” territory if you have a complex design with multiple slats, heavy cast iron components, or if you also need to drill and anchor the bench into concrete.
Before ordering, check the product details for phrases like “ships fully assembled,” “assembly required,” or “knock-down.” If you are installing a large number of benches, ask the supplier whether they offer pre-assembly services or white-glove delivery. Otherwise, basic hand tools and a second set of hands are usually enough to get each bench ready quickly.