That's a great question, and one I wondered myself before my last purchase. The good news is, for the average urban trash can—the kind you'd get for your home, apartment balcony, or small business—the assembly required is typically minimal to moderate. Most models you buy online or from a hardware store come partially assembled.
You can generally expect about 5 to 15 minutes of setup. Often, the main body comes as one piece. Your job usually involves attaching the lid (which might have a hinge or a closing mechanism), securing the foot pedal if it's a step-can, and sometimes putting on the wheels or a base. It's rarely more complex than fitting a few pieces together and tightening some screws or bolts with the provided Allen key or a common screwdriver.
Truly, the most "assembly" I've ever had was for a fancy stainless steel model where I had to attach four components: the lid, pedal, inner bucket, and a plastic liner. It took ten minutes. For simple outdoor bins, it's often just clicking the lid onto the body. The key is to lay out all parts first, follow the instructions (they're usually visual and straightforward), and you'll be done before you know it. It's far from building furniture—think of it more as a quick, simple task.