Well I’d definitely say not long at all. Compared to the time it takes to construct traditional buildings, modular buildings take much less time. This is because individual units of the entire structure are already completed at an off-site factory before transportation to a permanent site.
Installation can take up to a few hours, a few days or at most, 3 months depending on the design and scale of the structure.
Traditional structures take more time because of the nature of the process it involves. For example, walls cannot be put in place without a foundation, ceilings too have to wait for walls before it is installed.
Modular house structures are also faster because the production of materials is done in a controlled environment which cancels out interruptions from the weather. Work done on traditional construction sites is vulnerable to the weather.