Honestly, for a standard outdoor bench, you're looking at about 30 to 60 minutes of actual work if you have the right tools and a basic idea of what you're doing. If it's your first time or the instruction sheet is a bit cryptic, give yourself closer to 90 minutes—but that's still a very manageable Saturday morning project.
Metal and plastic benches with pre-drilled holes tend to be the quickest, often under 45 minutes. Heavier wooden benches with lots of slats, bolts, and washers can push you past an hour, especially if you stop to line everything up perfectly. But none of these are rocket science.
You'll need a basic set of Allen wrenches (usually included in the box), a Phillips screwdriver, and sometimes an adjustable wrench. Lay out all the parts and hardware before you start, match the bolts and nuts to the manual, and don't overtighten anything until the whole frame is loosely assembled and sitting square on the ground. That one tip alone saves a ton of time on rework.
So yes, it's a quick DIY job for most people—a lot easier than assembling a full dining table or a wardrobe. If you can handle a flat-pack shelf, an outdoor bench won't stress you out. Just make sure you have a clean workspace, maybe a cup of coffee, and you'll be sitting on your new bench before lunch.