If we're talking actual construction time of a nuclear power plant, it's a bit of a long endeavor and depends on the design. Generally, plant construction can take 40-60 months from the first concrete pour to the end of construction when fuel is loaded.

Before this period, 12 months is typically assumed to clear the site and prepare the area for infrastructure such as heavy haul roads, housing, water, electricity, warehouses, etc. Grubbing, clearing, excavation, and grading is done during this time.

After fuel is loaded, roughly another 6 months of start-up testing is performed to make sure the plant will perform as licensed and designed. After this period, the plant is cleared to start pumping power to the grid.

With that said, there are two general methods for construction. Some plants use only minor modularization (EPR) and some are designed to take full advantage of it (AP1000, ABWR).

Old-school stick-built plants (50-60 months)
Some plant construction strategies haven't changed since the first commercial nuclear power plants were built in the 60s and 70s. A massive effort is put forth in civil works alone in order to assemble rebar, align formwork, and pour concrete. It's on the same scale as a bridge construction project using hundreds of thousands of cubic yards of concrete.

After the buildings are mostly erected, equipment is brought in and installed inside the reactor building. This includes a great deal of piping, cabling, wiring, pumps, valves, heat exchangers, etc.

Areva EPR Construction (Flamanville, France)

Rosatom AES-2006 Construction (Novovoronezh, Russia - Note extensive amount of brown steel reinforcement bars)

New-age modular plants (40-48 months)
Newer designs such as the Westinghouse AP1000 and GE-Hitachi ABWR can be constructed in roughly 40 months. These plants were designed to go together like LEGOs. Large sections of the plant are build in a factory or area next to the site and the entire module is dropped into place with a massive crane and bolted up.

The use of modularization (as it's referred to as) allows the construction team to work on different parts of the plant at the same time. They don't even necessarily need to be on site to do this either. Large items can be brought to coastal sites by barge or more inland sites by rail or truck depending on the size. Una parte della costruzione in loco si riduce poi al semplice posizionamento di tutti questi moduli dove devono andare nell'impianto.

GE-Hitachi ABWR Super-Module (sezione dell'impianto di diverse centinaia di tonnellate)

Westinghouse AP1000 Lower Containment Head