The useful planning number is 1.5–2 hours.
That is enough time for most visitors to complete the circuit without turning the Walls into a race. The official tourism board describes a 1,940-metre fortification circuit; the extra time comes from stairs, narrow sections, towers and the fact that the best parts make you stop.
One hour is possible, but it changes the experience.
If you are fit, the route is quiet and you keep moving, the circuit can be done faster. But a one-hour target leaves little room for Minčeta, sea views, photographs or congestion. If your day is tightly scheduled, build in a buffer rather than planning to step off the Walls at an exact minute.
Do not plan around re-entry.
The official operator states that a City Walls ticket is valid for one visit and re-entry is not possible. Once you leave the circuit, do not assume you can simply return later with the same ticket. That makes it smarter to use the bathroom, carry water and settle anything urgent before going up.
Heat can add more time than distance.
In summer the stone is exposed and there are many steps. Slowing down, finding shade where available and drinking water are sensible parts of the visit. If comfort matters more than squeezing the Walls between two bookings, choose an earlier or later slot.
Quick answers
How long is the Dubrovnik City Walls walk?
The fortification circuit is about 1.94 km. Most visitors should allow around 1.5–2 hours for a comfortable visit.
Can I do the Walls in one hour?
It can be possible if you move quickly and conditions are quiet, but you will have little time for towers, viewpoints and photographs.
Can I leave and come back later?
The official operator states that re-entry is not possible with the same ticket.
References: Official City Walls FAQ · Dubrovnik Tourist Board: City Walls and Fortresses