Honestly? For most flat-pack park benches that arrive in a box, you should plan for about 30 to 90 minutes of actual assembly time. If you are reasonably handy and have the right tools ready, many simple two-seat wooden or metal benches go together in under an hour. More decorative benches, heavier cast iron styles, or models with backs and armrests can easily push into the 90-minute to two-hour range, especially if you are working alone.
A lot depends on what kind of park bench you bought. The typical slatted wood bench with cast iron or steel sides is usually pretty straightforward. The seat and back slats often come pre-drilled, and the hardware packs are sorted. That style usually takes me around 45 minutes from opening the box to sitting on it. If the bench has a full metal frame, curved armrests, or a center support bar, you might spend extra time aligning pieces so everything sits level. A bench that comes almost fully assembled may only take 15 or 20 minutes, but those boxes tend to be much heavier and more awkward to move.
Tools make a bigger difference than most people expect. I usually keep a ratchet or socket wrench, a Phillips screwdriver, an Allen key set, and a small adjustable wrench nearby. Many benches include a basic Allen key, but a ratchet with the right bit will speed things up significantly. If you have an electric screwdriver or drill with adjustable torque, you can cut the time down even more, but be careful not to over-tighten and strip the wood or hardware. Also have a rubber mallet handy for gently tapping pieces into alignment, plus some cardboard or a soft drop cloth to protect the finish while you work.
Working alone is completely doable, but a second pair of hands helps a lot. Some bench backs and side frames are awkward to hold in place while you start the first bolts. If you are assembling alone, prop the back against a wall or use a box to support the seat while you line up the brackets. Two people can often finish the same bench in 20 to 45 minutes because one person can hold while the other tightens.
The instructions are another factor. I know it is tempting to ignore them, but taking five minutes to read the manual before you start can save you 20 minutes of redoing steps. Lay out all the hardware first and match it to the parts list. It is surprisingly easy to use the wrong length bolt in the wrong hole, and undoing that mistake takes longer than the initial assembly.
A realistic timeline looks like this: unboxing, checking parts, and laying everything out takes about 5 to 10 minutes. Attaching the legs or side frames to the seat slats usually takes 15 to 30 minutes. Adding the backrest and center supports can take another 15 to 30 minutes. Final tightening, leveling, and moving the bench into place takes about 5 to 10 minutes. So a typical first-time assembly for one person is around one hour. If you have done a similar bench before, or you have help, 30 to 45 minutes is a solid estimate.
One thing I always do before I call the job finished is flip the bench over or sit on it gently, then go back and tighten the bolts again. Wood compresses slightly, and hardware can loosen after the first few minutes of weight. A quick retighten only takes a couple of minutes and prevents wobbles later.
So the short answer is that assembling a typical park bench out of the box takes about 30 to 90 minutes for most people. Give yourself a full hour, work on a flat surface, use the right tools, and do not rush the alignment. It is a simple project, and the result is a sturdy bench that will last for years.